The world is a dangerous place to live; not because of the people who are evil, but because of the people who don't do anything about it. Albert Einstein
On the positive side, at least AVG has a repair function
In these days of incremental updates, the pattern/signature files do get corrupted from time to time, due to failed downloads.
I recently had to fix a copy of PC-cillin (v14) that had that problem. I had to download the relevant files in their entirety, manually replace them and then run the update!!!
Incidentally the files in AVG are incavi.avm and upd_vers.cfg I believe.