This is for a computer with the following configuration:

Asus P5KPL-AM/PS motherboard
Pentium D 820 (dual core 2.8 GHz)
1 x 2 GB DDR2-800 Transcend RAM
1 SATA2 Samsung 160 GB HDD
1 PATA LG CD/DVD Writer
PS/2 generic Keyboard
USB Razer mouse

The computer was working fine till a month ago with a Hitachi HDD in place of the Samsung. But that drive died and I sent it in for warranty replacement. Since then and till last week, the computer had not been turned on. I then got hold of a spare but XPSP2-loaded Samsung drive from another computer and decided to get this computer temporarily running. However, when I tried to boot into XP, it didn't load. It got stuck at the first graphical screen. When trying Safe Mode, the file list would be updated up to diskio.sys and then freeze. If I tried to boot Kubuntu off a CD, I would get past the first menu, but then get stuck at the subsequent graphical interface in the loading process. A similar thing happend with a friend's Windows XP-on-a-pen-drive.

I switched the RAM into the other slot, but no luck. Then I booted the Ultimate Boot CD and ran Memtest86+ and a couple of the bundled CPU stress tests but these detected no errors. Some searching on the Internet brought up the notion of the CPU failing to switch from "real" to "protected mode".

I'm hoping to get a fix on what the problem is and what I can do about it.

Edit: I've flashed to the latest BIOS, but that doesn't help. Also, Knoppix LiveCD also freezes. I notice that the LED in my mouse goes off at the time of freeze, but trying to boot without the mouse connected produces the same result.