on access didn't run

i am using avast since 3 yrs…but now i face a probelm on my computer… am using windows xp Sp2 and avast 4.6 home editon. problem i got is that avast’s onaccess protection is always automatically disabled… I cann’t enable it… when i try to enable it message will come as “OPERATION CANN’T BE COMPLETED”…what is it… can any body solve it… if so mail me on thapaliya@gmail.com

Do you have any other antivirus in your computer?
Did you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair). You must be connected to the internet while repairing.

If this does not help, can you uninstall / boot / install / boot again?