Problems booting old computer

Problem:
My brother's computer is 5-6 years old and stopped booting once we tried to remove his dvd drive (we assumed it was not getting enough power to run properly).

System specs:
300W PSU
Athlon 900 MHz
768MB DRAM (100 MHz)
GeForce 2 MX
DVD-ROM 16x
CD-R/RW 48/16/48
Sound Card (I think its SB AWE32- it's in the ISA slot)
USB card (with a Logitek gamepad attached)
USB Linksys WLAN connection point (don't know the technical name for this thing)
IWill KK266 Motherboard

Detailed Problem Information:
He also used to have a modem, but removing it improved the stability of his optical drives (they used to occasionally crash his computer when reading disks- mainly during intense activities like installations. Plus they were finicky about opening when pressing eject button), which is why we thought it was a PSU problem. So we decided to remove the CD Burner also, and we set the DVD to master. Now Windows XP won't boot. It gets to the step where the "Welcome" screen should show up but restarts instead. Booting in safe mode freezes the Windows bootup before it gets that far.

Any ideas on what to fix or need more information?

 

Try removing the USB card and using a PS/2 keyboard and mouse.

 

Okay tried that and no change. I think I get the idea of your suggestion, so we tried removing all non-essentials (both optical drives, USB card, and SB card) and booting. Still no change.

But there is a new thing.
His screen bleeds white during the RAM check, and it spreads to cover the entire screen until the Windows Logo with the moving bar underneath appears, at which point the image fixes itself (going into the BIOS setup also fixes the screen). Apparently it has done this before he was even having trouble with his boot-up but it didn't happen all the time. So my question is, how does this change the problem? I thought it may be something burning, but I don't notice any odors and neither the video card nor CPU were hot to the touch once we turned it off (the BIOS CPU temp sensor said it was about 46C, no sensor on anything else).

 

Might want to run a RAM test:

Memtest86 - A Stand-alone Memory Diagnostic
http://www.memtest86.com/

Windows Memory Diagnostic
http://oca.microsoft.com/en/windiag.asp​

(Let 'em run for a while, [either one]. You should not get any errors).