If you've tried the virus and malware scanners without success - and you are running out of knowledge - (despite agreeing with VHG in principle) I think the time that VHG hates is here --- time to crash and burn.
either that or accept you've had it ..... throw it away .... buy a new one!!! ;-(
alternatively .......
download magical jelly bean - v1.51 is fine
http://www.magicaljellybean.com/keyfinder.shtm l
or version 2 (beta) allows you to change serials
http://www.magicaljellybean.com/beta/
using either one on your machine get YOUR XP serial number
Providing that your mum's version is exactly the same
install using her CD .... but enter your serial and activate.
if your mum's discs are restore - use them (don't connect to internet) - use version 2 to change the existing serial to yours - THEN activate it .
Personally I rebuild my machines every 12-18 months
(but that's my choice).
support guys generally work to a 20 minute rule .... if you can't see the light after that time it's generally more economical to use the rest of the time rebuilding.
most of the big guys charge �90 an hour ... while no one likes to be beaten .... it's worse to spend �180 on a machine that's 4 years old