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system file getting loaded on boot.
This is only a first reading of your prob.Other suggestions may follow.

Clean out all your rubbish using CCleaner (freebie)
Next cleanout your registry (Auto) using RegSeeker (freebie)
Now run a full AV scan using both AVG and AntiVir (freebies)
Now Use SYSTEM RESTORE and restore to a date earlier than your virus trouble.
Now test the machine.
If this does not work, uncheck SYSTEM RESTORE and do another full antivirus
scan. This automatically deletes the System Files where a virus could hide.
Now re-check the System Restore box.
Now re-start.
Now tell us if you get a good result.

If the above fails to solve your prob, you might need to do an MBR.
Do you have the original XP CD ?
Fingers crossed.. *LOL*