[IHAVEAVIRUSHELP]

I have windows xp media center edition, and at the moment i need help with a virus called:

Rootkit.Win32.TDSS.c in my 'System Memory'

and i need it removed... I need to remove it manually or without my original antivirus software; becuase everytime i open
it to disinfect or delete it: it opens up windows saying that some of my programs are disabled (however they arene't)
then afterwards i reboot my computer and the scenario continues...

I need to delete this..
and please explain this to me in easy terms for a 15 year old to understand as i am not highly computer literate.

Thank you