Which is your Windows? 98, Me, XP?
You can boot at Safe Mode (F8 while booting) and scan with avast from there.
Can you pist the name of the file infected (and its path)?
If Avast is corrupt can I reinstall it over itself to repair it?
Yes, or 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.