I also see it as a hardware/driver problem. But the fact it crashes with SONAR/Piro only can be helpful.
 
I will first try to remove all audio and MIDI devices from SONAR configuration. Even better disconnect both devices.
 
If you still get crashes, you can try to reproduce it with some graphic+memory+CPU intensive programs (for example 2D/3D benchmarks or PC benchmarks).
 
If you do not have crashes without audio, try to work with completely different audio hardware/driver. For example with MoBo build-in audio under ASIO4ALL.
 
At this point it should be already clear either the problem is in MoBo or not.
 
SONAR X2 does not do anything special on low level (it will not work under Wine in Linux then, and it does for me).

Is it possible that you have some 32-bit plugins loaded into projects, in a 64-bit Sonar?  Sometimes third-party 32-bit plugins just don't like running in 64-bit Sonar, with either the default included bridging software in Sonar, called BitBridge, or with the external paid version from some other company, called JBridge.
 
Lots of folks have switched completely over to using nothing but 64-bit plugins, and Sonar certainly seems to run smoother and more stable with all third-party plugins being 64-bit.
 
Just a thought.
 
Bob Bone
 

Is this the older 1604 or the newer 1604i ? 
I still am thinking the Mackie drivers. Just google and read about them you'll see even people with Mac's had issues when using certain DAW platforms like Pro tools. 
And if yours is the old version then you are using XP 32 drivers.. 

There's no need to get anything special in regards to a video card. As you mention, you are doing just fine so far with the built in graphics your system has without any grief. There are a couple of low end options from both NVidea and AMD that have no fan and therefore add no noise to your recordings.
I have a GT240 that works just fine. I know Sonar doesn't put too much stress on the video components of a computer but I think it's still worth the effort of giving your system a video card to take those tasks off the CPU and give them their own dedicated driver; it definitely made a difference to my system, which is quite low specked.
It will take one of the parts out of the equation, and one that is quite a simple and cheap option to try. Good luck and keep us up to date on your progress.

The audio interface is the Mackie 1604 mixer with firewire card added.  This was the very first generation of mixer/interface that Mackie built.  It is not the newer 1604i.
 
Mackie has only released two drivers for this interface: ver 1.0 is for use with Win XP and ver 1.07 for Win 7 and later.  I believe Mackie says use the ver 1.07 driver for 64-bit OS.
 
Yes, the problem could be due to the Mackie driver, tho' again, I have not had any problem using Sound Forge and Sound Forge makes use of the driver.
 
So I'm in a position of possibly having to replace my audio interface (if I can determine that the interface is causing the problem), or replacing Sonar (if I can find a DAW that will work with my existing computer and interface (as Sound Forge seems to do)), or having to replace a MOBO or a processor or a HD (if I can determine there is a defect in a hardware component).  And of course, we still don't know for sure what exactly is causing the problem.
 
So I'm thinking my game plan is to
1) do the VST rescan and check for HD firmware updates as suggested by AlexS
2) installing a dedicated graphics card, as suggested by a few people
3) if problem persist, buy a inexpensive audio interface to use instead of the Mackie
 
Sigh - and the possibility remains that I could do all of that and still be having the same problems.  

It occurs to me that my "to-do" list should include trying the demo version of Cockos Reaper and see if my computer crashes/freezes while using that (Cockos Reaper because the demo version is fully functional for 60 days - a good long testing period).  Maybe do that before anything else.
 
Given that I only see the freezes/crashes when using a CW product, it will be interesting to see what happens when using a different DAW software.