My wife's computer keeps rebooting and I am at a total loss! It's a P4 3.2 on a Microsoft motherboard (not sure the model), 3GB ram, firegl x1 video card, creative live 5.1 sound card, running XP with Microsoft live virus protection. This computer was originally a server at my work until they were obsoleted and given away. The problem first started coming when you were in windows and outlook (I know this sounds really strange) and you did something that caused the computer to make the 'ding' sound. It was completely repeatable! The computer would immediately after the 'ding' put the monitor to sleep and the mouse would turn off but he keyboard would stay up (caps and num lock light would change with key press). I would have to kill the computer with the start button and reboot to get it back up. Soon after it started happening sooner, not corresponding to anything maybe 5 min after booting into windows. I tried to run a disk scan but it would get most of the way thru it then reboot and start again. Now it gets to the windows splash screen and reboots (over and over). It will even do it if you leave it on the bios screen too long. I swapped the power supply, pulled the video card, verified all fans are running and cleaned all the dust out (no overheating). I want to try a different video card and pulling out ram but I'm running out of ideas.
You've done all the right things to give it a chance to work properly.
It becomes more of a challenge when it can't even stay in the BIOS setup without rebooting.
This make two items suspect. The video card and the system memory.
You have a good idea to try different ram and video cards, that appears to be all there is left to do and it appears that's the last two things you wanted to try, so keep going. If there are two or more mem cards, try just one, then the other.