Computer stops responding for few minutes

I have a fairly new computer. AMD 9950 Quad Core. 2 monitors, 1 video card (128 bit) PCI-E x16@x16. Windows 7 RC 1 - 64 bit but have the same issue when using Vista Ultimate 64 bit and 32 bit. My issue is... while working in a program it will stop responding (at top of window I get message that the program has stopped responding). Computer activity light is solid red and I have no idea what it is doing. I go to another program and it responds when I first enter it, then it stops responding in the same manner as the above program. Then all programs stop responding. I can't ctrl-alt-delete. Can't restart. It's like the computer is frozen but it's not. In a few minutes, all is fine.

I bought this barebones computer in November of 2008. Sent it back once, a new CPU was put in even though the company said all checked out fine. I called again today and the guy said it might be memory (8 GB). I've tried each stick separately with the same results. Power supply is 600 watts. Does anyone know what might be happening? Thanks a lot for any help for my problem.

flyin, thanks for the reply. I did a chkdsk /F on my HD and all seemed OK. I've been using 2 monitors with 2 different cards. Switched to just one of them and so far, so good for the last few hours. I'm hoping that was the problem. If not, I'll do some more troubleshooting. Thanks again.