Same Problem with Both AVG and PC Tools AV

I had AVG (free) as my main anti-virus program, until about a month ago, it kept giving me this error message diring a scan: "AVG Scanning Core Module has Failed" or something like that, then the scan would stop.

I then installed PC Tools Anti-Virus and did scans with the program without any problems, until today, it gave me a "error 104" message and the scan won't even start.

Please help me,

Thanks.