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.