I have a separate computer for most internet stuff (like this).
I run a script before running the DAW that disables all the internet connection and securities that go along with it - and have since gone to a wireless USB wifi which I pull completely from the machine when not in use.  And I've uninstalled all mail clients. The only time I allow internet to run is during an install for challenge/response/software updates. Windows updates, I've become judicious about which ones I allow and keep it off most of the time. This all seems to work well enough, but now it's really getting time for me to venture into Core 'Lasso-ing' (Directing which cores are used by which programs & OS). I've been a bit lazy in getting on that.
 
Oh, and the basic MS Security Essentials/Firewall work fine for all that.

Can't knock MS Security Essentials. I use it for the casual PC's here at home.
 
As for back-up I, use Retrospect - besides the workstation it's running on, it comes with 5 additional remote client licenses too. It's backing up four desktops and a laptop. It's also cross-platform, so if you have Linux or Mac boxes, you can back those up too.
 

A + for MS Security Essentials (since the OP is on Win 7)
 
Regards

Here's the thing, and I'm not saying Norton or the other AVs have not improved as far as efficiency because I'm sure they have, but what are these conclusions being based on? I'm genuinely curious so don't take it the wrong way. Are you monitoring the performance in comparison to MSE? Tech articles and shootouts on the subject? Or is it because you don't notice as many problems? I say this because the difference may lie in the fact that computers and internet connections are much faster than they used to be. For example on my old underpowered laptops I tried a few different AVs like AVG, Norton (free version) and Spyware doctor before learning about Avast. AVG and the others made the systems almost useless (this was however about 5 years ago). They didn't just freeze up during the definition updates they were just horribly slow in general. Once I installed Avast everything was much snappier and the only time I really had any lags was if I was trying to do something like load a bulky webpage or play a video and the definition updates started loading as well so I'd just wait for a few minutes and things would go back to normal. Once I bought my new laptop those lags completely disappeared and I don't even notice Avast doing anything unless it happens to snag a bad page or something is trying to screw around with my system (which rarely happens because of ABP and NoScript and the fact I don't scrape the underbelly of the internet). I haven't tested the other AVs on this system aside from the "free" MaCafee (sp?) that came with it (which BTW DID make the system slower) so I do not know what, if any, kind of drains they'd cause so seriously I am just asking out of curiosity.
 
MSE has for the most part consistently beat out the rest of the pack as far as resource issues and ranked very high for virus prevention (although they've slipped a bit in those rankings but last I checked it was only some of the more expensive paid AVs  topping them like Kapersky).
 
Basically if it works for you guys that's great but you may have more powerful systems than others and/or not be doing stuff that is as resource intensive as other folks. The true measure of these types of things are benchmark tests. I'll admit I have not viewed what the various reviewers/testers have come up with in the past year or so and some of those can be skewed due to the testers not wanting to PO their advertisers. All I know is last time I checked MSE was right at the top of the lists, is free, was recommended by Cake tech support, does not need to be turned off or have any settings changed and seems to work on my system. I did have an odd thing happening a little while ago that I thought might be a virus but after a ton of scans with the programs I mentioned earlier I found nothing except some false positives. I think it may have been some kind of BIOS warning but it has disappeared now so IDK.
 
Again not saying I'm right or you guys are wrong. I just think it's important stuff to consider because virus protection, especially on a DAW, is a pretty serious topic and deserves appropriate attention to not only avoid viruses and malware but to make sure any problems with Sonar or other programs aren't because the AV is doing something funky. And that funkiness doesn't even necessarily need to be resource related. It might look at legit programs and cause weirdness especially considering many of the things we use are pretty darned obscure. That's exactly how I got my false positives when running my scans a few weeks ago.
 
Food for thought. Hope you've all been well. Cheers.

So far MSE has been transparent after using it a couple years. I just recorded a 2 hours straight through 20 track at once session last night including having AC7 connected. No problems including mix down. And my computer is pretty old now. MSE has been much better then when I used to use ZoneAlarm and Norton. But it's been many years since I used Zone Alarm and Norton. The computer is always online and I have done zero optimizations for a DAW.

Thank you all for your helpful feedback; this forum is fantastic. 
 
Lots to think about here, but I'm encouraged by the reports of Microsoft's Windows Defender and Security Essentials in Win7.  I suppose I've been using Windows Defender without really being aware; I'll check the next time I boot up, but if it runs by default I'm probably running it.  I'm also NATed behind a firewall, and I don't have any oddball ports open on the router.  I don't run MS Office or anything like that, it's essentially all audio/video applications on this machine.  I'll occasionally read/send email via a webmail interface, but I don't open any link I don't trust.  Hopefully, if I browse responsibly I should continue to be OK...
 
@sharke, thanks for the tip about the AV folder exception list for the plugin folders; I can always use another 60 seconds, seriously.
 
@SuperG, backup, which I'm not currently doing (I know, I know...), is my next adminstrative chore for the DAW.  I'll be sure to check out Retrospect, thanks.

I've been using sandboxie for a couple year, with no problems, though I do run something like adware search and destroy

Quite a good Q&A page explaining Windows Defender and MS Security Essentials on Win 7 and 8:
 
http://answers.microsoft.com/en-us/protect/forum/mse-protect_start/windows-defender-and-microsoft-security-essentials/5309cb8d-02e1-40e8-974f-0dcedb9ab9fd
 
In light of this I think I'll check that I have SE on my DAW machine :-)

These days it doesn't matter. Its not even thought upon with todays Pc's. Install what ever antivirus program you want and nothing will happen to sonar and its performance.  Its an exaggerated point that got taken to far. I even bought into it for awhile until I actually tried some of the so called heavy hitter, CPU intensive antiviral software.
 
If an antivirus program is hindering you from using Sonar, you have got bigger problems
Cj

Caveat: AntiVirus software can affect the performance of Sonar. If the A/V software is running any kind of scan on your actively working hard-drive, Sonar will certainly, and profoundly be affected, performance-wise. I highly recommend setting scans to run at a time when Sonar isn't in use. The reason is that two software applications are competing for I/O, and your system must thread these by interleaving the I/O throughput. A/V will run as fast as your system will let it, although Sonar, may only need a fairly consistent I/O throughput. Sonar may not affect the A/V scan times, but the A/V scans can certainly affect the I/O available for Sonar.