PC Freezing/restarting

My PC is freezing/restarting often when i play games/watch movies and less often when i browse web.

I changed PSU and Cooler, no effect, pc still freezing/restarting

I have reinstalled windows, no effect.

When it freezes, i have very bad sound and sound changes a bit if i change video card fan speed.

My PC:

Motherboard - Intel Desktop Board D865PERL.

cpu - Intel pentium 4, 2800Mhz.

ram - 1022mb ram DDR400.

video card - Ati radeon x1650.

PSU - 400W.

Is there any way to stop this random freezing and restarting?

Remove/disconnect everything except 1 memory stick and harddrive.
See if problem is still there, if so try other memory.
If not, probably the psu. "400 Watt" means nothing, it could be the peak it can supply for a short time, but it might not give 200 Watts solid without power drops.

And of course check your cpu temperature, re-do the thermal paste between cpu and cooler (once a year), clean all the fans. Also check for loose mainboard fitting screws, reseat vid. card - soundcard, etc.