Used AVG for about six months and found it slowed down the opening of files to the point where you found yourself screaming (particularly if you were on the phone and trying to retrieve files for reference - always found myself apologising to the person at the other end of the line for interminable delays).
I've now changed to avast! and guess what: on the first scanning of all my files (which were regularly scanned by AVG), I found no fewer than 46 events of threats, worms and trojans!