'Close Encounter' of the MUP.SYS file at WinXP safe mode boot

Bad little aliens have invaded your computer and sent a message 'MUP.SYS'. Two days ago my computer freezed at the Windows boot screen. Restarted and tried to go into Safe Mode, the magic filename 'MUP.SYS' greeted me. Loading in safe mode will get me stuck at mup.sys, loading windows normally freezes at the screen itself instantly. Didn't know I would be in for a torrid and agonizing night trying to find the problem.

Here's what I did trying to isolate the problem :

1. Hard disk: Took it out and slave it on another computer. Scanned with antivirus programs, scanned for hard disk errors and bad sectors. None is found. HD seems to be working fine. Take note, I did not even try to repair Windows with the WinXP cd. Then I took a HD from another computer and connected to the problematic PC, the same thing happened and hello 'MUP.SYS'.
   
2. Next, I loaded the default settings at the BIOS. Tried resetting CMOS. Swapped and changed the cables. Disable USB at the BIOS. Doesn't seem to work at all.
3. Swapped my spare PSU ( power supply unit ) in. Took out the sound card, and network card. Changed different keyboard and mouse. Unplugged my DVD and CD drives. Tried to look for leaky or bulging capacitors on the motherboard. None was found. Made sure every cable and card is nicely fitted in. And still hello 'MUP.SYS'.
   

Finally, I took out the second stick of RAM and left only one fitted in. Started the computer, it went to the prompt screen where you can choose to load into safe mode or load windows normally. So, I chose Safe Mode, and a miracle happened it actually went past MUP.SYS while loading! Hooray!!!! Boy, am I relieved!

The RAM which I took out, works fine on another computer. Shows how a faulty RAM slot can do to you. Hope I'll never have to go through it again. This is being discussed extensively at this link here, if you ever need help or trying to troubleshoot the dreaded MUP.SYS syndrome.