can someone help please. i resently updated my virus checker(AVG-free edition v.6). it found a few viruses and killed them except that now i keep getting this same pop up box which says as follows,

AVG Resident Shield
Virus
Trojan horse Backdoor.Agent.BA
is found in file
C:\WINDOWS\system32\resepad.dll
To remove this virus, please run AVG for Windows.

I've tried running Avg again and it finds this virus, but it says it can't heal the file and it can't delete it, so everytime i open a window or start and app, this box pops up.
Can somebody please tell me what to do.