What is your OS?
What avast! version and VPS file (virus database) number, e.g. 0436-4 (see about avast!)
What was the virus name, what was the filename, where was it found
example (C:\windows\system32\infected-filename.xxx)?
You can try the move option and see if that is allowed.
Try a repair of avast. Add Remove programs, avast! Anti-Virus, Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.