I ordered a bare bones system a few months ago, and things have been working great. Recently the computer has been resetting though. I discovered the socket the power strip plugs in to was very loose, so I switched sockets and things seemed better, until eventually the machine suffered another reset. In optimism, I replaced the power cable, but that didn't help either (I figured it wouldn't, but oh well).

The motherboard is MSI P4M900M2-L Socket 775. Not the fanciest motherboard ever, but I like it.
The ram is 2 x US Modular 512 MB PC4200 DDR2 533MHZ.
The chip is Intel Celeron 356 3.33 GHZ 512K 533 FSB 775.

I know Celeron's suck, which is why I'm getting ready to replace the chip with a nice dual core. However, I'd like to figure out why in the hell the machine is suffering resets.

The system temperature doesn't get out of hand. Sometimes it will reset while I'm viewing the BIOS menus. Sometimes it will reset 5 minutes after I turn it on. The resets seem to be happening more frequently. The fans are all working, all cables are firmly connected (double checked them about 2 minutes ago), etc.

Any suggestions as to which part is most likely the most guilty: power supply, mother board, or ram?

Extra info about the system:
XP Pro 2, all updated drivers, all patches, etc...
Seagate 80GB Serial ATA HD 7200/8MB/SATA-3G
Liteon 52X32x52x CDRW Internal Black
GeForce 7300 GT 256mb RAM (PCI-E)
Netgear PCI WG311T (Version 3)