Your Computer Guy needs to know that AVG is now (and has been for a long time) a part of Avast Software.
You would need to ensure that your previous AV installation was removed or there could be conflict.
However, 24-30 hours doesn't seem anywhere near normal (but I have no experience of trying to scan 5TB+ of data), have you made any changes to the default settings of the Full Virus Scan (also was this the scan you ran) ?
There are some settings to gather information on the scanned files (placed into a persistent and temporary cache) so they won't be scanned unless the file has changed (persistent cache) or there has been a new VPS (virus signature) update. This does take time over about three scans to build up this information, but would save on scan time on future scans.
Assuming you did the equivalent scan with AVG Pro, how long did it take (10-12 hours, given your overnight comment) ?
Do you do these scans regularly ?