Freezing and crashing all the time

I have two computers:
*Compaq Presario SR1265CL
and:
*ASUS P4P800SE (I think)
I can get all the specs if necessary.

Both of them have the same problem. Whenever I play BF2, watch internet videos, or try to download stuff, the computer crashes, freezes, or reboots. I tried changing the video card a bunch of times (like 5 different cards), swapping the power supplies between the two computers, changing the ram, running memtest, reinstalling the OS... nothing works. So I figure the problem has to be one of the following:
*both motherboards are damaged
*both power supplies are damaged
*all the video cards are damaged

Does anyone know what could be causing this? Thanks in advance!

Have you tried updating your system BIOS ? It might be because of that , or try reformatting it.

Both machines have the latest BIOS revision, and yes I've tried reformatting.

Hmm , latest BIOS .. so your motherboard might not be damaged , since you think that your video cards are damaged , why don't you try updating them ? try cleaning your registry using ccleaner , or try disk checking by typing chkdsk/r in command prompt , try defragging your hdds' ..

Yeah, not really helpful, as I said I reformatted, reinstalled XP, and tried every AGP card I own; Thanks for trying though.

It might be important to note that the USB mouse and keyboard sometimes stop working and I have to unplug and replug them. From what I hear, that usually means a bad PSU. Can anyone verify this?

The Same thing has happened with me on windows 7 along with blue screens .. but my win 8 works perfectly.

On your earliest board, may be bad caps.

And even with bad caps, it's possible for the caps to be visibly fine.

I would check out this place:

http://badcaps.net