Exactly what were you trying to scan, file (what type, text, html, zip, exe, dll, etc.), folder, partition, HDD? If you can provide the path, e.g. c:\windows\system\scanthisfile…, etc.
I would suggest that you 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.
Neither can I explain it, but thanks for letting us know, it may possibly help someone else experiencing a similar problem and have PowerDesk installed.