is your computer set to "shut down when a certain temp is reached" as a safety precaution in your system bios? maybe your cpu is reaching a pre-determined temperature (turn this function off) and shutting down the computer, during this intensive task.
how about the "sleep" function on your computer? do you have a hot key on your keyboard which you are hitting during the game that makes your computer shut down?
make sure all other programs are shut down before you log onto CS and try rebooting before you play to clean out the sys memory.
speaking of memory, do you have enough ram?
you may also want to check your screensaver/power saving settings in the screensaver, advanced tabs in control panel. your hdd might be going into hibernation during the game, which I find unlikely due to mouse movement, but it's possible.
worst case scenario, I would re-install CS (and all those maps you've downloaded, uggh) and try again.
another cause might be you're on a slow connection (high latency/ping) and the servers are kicking you out.
hope this helped, and that the problem is solved soon!
maybe I'll see you online sometime...my handle is "brAZen" and I play from NH.
~Branson