With the greatest respect to people, I think there is a habit of attributing problems to the wrong causes, and then superstitions build up around those associations. Yes, sometimes merely installing software does leave it running in the background. And yes, software running in the background can take up memory and CPU time which may slow down your computer's performance. But generally speaking, other non-audio software is not going to change your audio drivers, it's not going to change your MIDI or audio inputs or outputs, and it has no way of inspiring plugins or Sonar to crash. I have myself experienced the rare occasion where another piece of software was legitimately affecting my Sonar use - and that was because it is a rare piece of software that does something quite specific with the graphics hardware. I have a ton of other things running (including things I would recommend others switch off, just to put people's mind at ease) and nothing interferes.
 
And sometimes, I think people bring it upon themselves by taking the superstition too far. "I leave my PC disconnected from the internet at all times, except every 6 months I reconnect it to download Sonar updates". Don't do that. Most likely you're suffering with broken or suboptimal OS software during that period, sometimes including drivers for your hardware, and when you do finally connect to the internet you have months-old security flaws in the software which make you much more vulnerable to viruses than someone who is connected 24/7 but accepting security fixes as they come.
 
The main thing I would suggest for multi-purpose PCs is this. Wait until 2 or 3 minutes after you've booted up, then open up Task Manager with Ctrl-Shift-Esc. Click on the 'CPU' column, and watch for a few seconds. Note any programs that keep appearing at or near the top of that list, and ask yourself (a) Do I know what this program is? (b) Is it essential I keep the program installed? (c) Can I at least set it to not start up automatically when I boot Windows? Answer those in order, considering removing the program or making it not start when Windows does. Then click the Memory column and do the same. (On Windows 8 you can do this for Disk and Network access, too.) Taming the top handful of culprits will be a decent use of your time.

Yup. Process monitor and process explorer are you friends! Learn the tools and try to find out what is going on... See what is resident, check what is writing to the hard drive etc...

I still get problems, many of which stem from my lack of understanding a "feature of Sonar", others are bugs.
 
But I can tell you it has really made my life better to have upgraded my interface to an RME Fireface UCX. I tried the Rolands and Tascams and Midimans in the past, thinking the RME was way overpriced and unnecessary for my level. But wow what a difference in sound and reliability. At first, the RME was not working so well, but after I disabled the built in soundcard that comes on the motherboard, I got the low latency, reliable, and pro sounding computer I wanted.

Johnbee58
Man, can I relate!
It's getting to the point where every single time I try to work on my music I only wind up frustrated with another software glitch.  It's getting to the point where it's not worth it.
 
Time to give up music and take up painting.  Seriously!
 
John B.

 This will always exist unless things become more proprietary. Reason has never crashed but the limitations are many. Now people have stability issues since they added rack extensions.  People claim Macs are stable but again there are plenty of limitations.
  PC gamers go through the same frustrations as well.
 

I have been running cakewalk for Ohhh  10 plus years in all of its iterations  I currently run x3 and Platinum in a production environment whilst running ethernet antivirus blah blah blah etc etc.  With no problems other than the usual bugs I would look at your computer config and make adjustments to hardware and such before blaming Sonar most likely you have a poorly configured workstation not ideal for audio production If you are interested in changing that pm me 
Cheers, Dave

Thank you all for your comments. There are so many differing opinions and solutions and results here that what I'm coming away with is what one poster said which is there's just no way to know. I was telling my wife about my troubles and that I might need to do something drastic like get a new machine/upgrade/etc, and she reminded me that I've been singing this song for years, and I upgrade, and get new machines, etc., but I always have these problems. I think the big issue is that I'm not invested enough in the software/hardware side of it. My music is really complex and has to be release-quality and I'm doing it all by myself and I have a day job. It's all I can do to keep the music parts straight in my head from day to day without adding in the computing. 
 
So, after all this, I just don't know what I'm going to do.
 
Starting a new thread which is an offshoot of these troubles.

Sonar has Mix Recall now, so I would suggest dumbing down the mixing process.
1.Do a barebones recording/playback setup w/ tracks only, output to a Master.
2.Click the big "FX" button on the Control Bar. This will (hopefully) eliminate any
   crashes from effect plugins or VSTi output.
3.Save the mix as "Barebones" in Mix Recall(by the way, the Mix Recall dialogue
   has a lot of this functionality[fx, et al] built in).
 
Repeat the process with Sub Busses, FX on, Time Based etc.
 
Hope this helps
 
Mel