Windows XP System 32 infection removal problem

Question

Hi,
I have a virus infection (Win32 Agent) in Windows XP System 32 folder at c:windowssystem32winNt.dll which will not allow me to delete to remove it.
My antivirus (AVG) removes the infection initially but everytimr you restart Windows it comes back.
Any ideas how I can safely remove the infection without messing up Windows??
Many Thanks