On the antivirus - some folks (I do this) add the Sonar and plugin paths to the Exclusion list in the antivirus software, and that way it won't keep trying to scan all the sample files and helps performance.  That way, you don't run a risk of forgetting to turn it back on.
 
Bob Bone
 

Here's a few things I do to keep my machine as much a dedicated sonar box while also using it for other stuff.
 
First, I use portableapps.com
 
Portable apps are apps that do not install on your computer, don't add or make any changes to the registry, and when you shut them down don't leave anything running. Just go to portableapps.com and you'll find many of them to choose from. You can also, if so desired, install all of them to a USB key and they will work across multiple computer and be totally removed from any computer when you remove the key.
 
Second, the only thing I install on my computer other than sonar and associated things (audio drivers, other plugins, etc.) is virtualbox. 
 
Virtualbox is also free and can be found at virtualbox.org. Virtualbox allows you to install a virtual machine on top of your existing machine. I build a virtual machine and run everything that I need to on that virtual machine. It actually winds up being what I use about 75% of the time, pretty much any time I'm NOT doing music. I just leave the virtual machine running and install everything I need/want in there. 
 
When you shut down your virtual machine, the base machine doesn't have anything installed on it (except virtualbox) and sonar/related.
 
You can also go here http://www.blackviper.com/ and find services you can disable. I use to do that, but don't bother any more as things seem to run pretty good for me these days.
 
gabo