I have this same problem, and I was about to post about it to I've researched it a little...
This problem occurs in all games
TF2 Version= Low OS Paged Pool Memory
Zombie Panic= Screen just goes Black
Only ones I've really looked at...
From what I've discovered the problem is that the computer doesn't have enough Virtual Memory
I was reading on the steam help site, that his problem mainly results from certain programs that hog all of it, which means it's most likely not that your computer doesn't have enough memory.
The also commented that the main programs that cause this problem, are Anti-Virus Programs.
I downloaded a trial of Norton 360, I started to notice the problems maybe a day after I downloaded it, I've tried to delete it,but it has some built it protection that prevents me from using Add, Remove Programs on it, and it has protection so I can't directly delete it out of my C-Drive.
You can also try and update your Video Card Drives, which in some cases has appeared to fix the problem.
If all else fails, you can reinstall windows which I personally plan on doing if I can't figure out how to fix it, because Norton's giving me problems.
You can also try and do a system restore, which will return your computer back to a point before the problem occured, for me this won't work, because once again Norton won't let me return for some reason
.
You can also try cleaning up/defragmenting your disk which will give you more space on your computer, and hopefully make it run more smoothly.
