Windows 7 freezes

Hello,
I have a problem, my pc freezes randomly when playing games and sometimes when doing something else (like watching a video or just surfing the net). No BSOD, it just freezes with an annoying looping sound. Usually it freezes in the end of the round in several games (mw2 and bc2), but sometimes randomly. When testing stability with prime 95, I ran it with no errors or freezes for few hours, when I stopped the test, the pc froze immediately. Now when I run prime 95 for five minutes and stop it, pc freezes every time. I tried the same in windows xp: played a few games for few hours and ran&stopped prime 95 - no freezes. I have been using windows 7 since launch and didn't have any problems until a week ago.

What I did trying to find and fix the problem:
Checked the temperatures: everything is fine. All temps are below 60 C.
Checked my RAM with memtest 86. Ram is OK
Reinstalled windows 7 both x64 and x86 several times.
Reflashed motherboard BIOS.

I don't know what to do now, can someone help me solve this problem?

My pc specs:
CPU: AMD Athlon 64 X2 3ghz
Mobo: Gigabyte M61PM-S2 (rev.2.0)
GPU: Gigabyte 9600gt 512MB
RAM: 2gb DDR2 667
HDD: 500gb
PSU: 400w

Thanks in advance.

Crispy said:

I suggest you get a new power supply at least 500W+

Yes that may be problem. My PSU is getting hot sometimes and it is almost at its limit in my pc. Do you know why my pc doesn't freeze in xp? If the problem is hardware related it should be also present in xp. Or does windows 7 require more power?

did you solved the problem?
if not -what antivirus do you have?

userme said:

did you solved the problem?
if not -what antivirus do you have?

I didn't solve it yet and my antivirus doesn't matter really (its nod32 now if you wanted to know) because It still freezes when no antivirus is installed. I think will buy a new good PSU, so I will be able to use it in my future pc build I'm planning this summer.

did you had AVG free installed before?
i think it's not the PSU cause playing in XP take resources from you hardware just as win7.
there is an option do catch the freeze with forced BSOD and get some details about the cause.

userme said:

did you had AVG free installed before?
i think it's not the PSU cause playing in XP take resources from you hardware just as win7.
there is an option do catch the freeze with forced BSOD and get some details about the cause.

No I didn't have AVG installed, and with each win7 reinstall I formatted my hdd partition completely. The strange thing is that I get freezes not on full load, but when the load quickly goes down. When I stop prime 95 or in the end of the round in several games. Maybe in windows 7 the power is managed differently. I still think its my PSU, I have had several problems with it earlier. BTW how to force BSOD and get the details?