I’m in the same boat as WillFred. I too have a windows Vista 64 bit machine.

I am having this same problem beginning today. The previously mentioned solutions produce the same results as WillFred noted; “File System Shield provider not found” when I try to restart the Avast, and “Error processing package. Please use full update” when I try the repair option in the control panel.

This seems to coincide with a recent Avast update, but my windows 7 laptop, which is also running Avast is having no problems.

I ran MalwareBytes Anti-Malware, a full scan and it found no problems.

To answer another question someone asked WillFred, I’m using Avast Internet Security