As expected, given my experience on my other PC, the first result is a general performance increase - not huge, but noticable - and a BIG improvement if I want to run other programs while a whole-system anti-virus scan is in progress; loading time is maybe a quarter what it was with Norton. Mail delivery and receipt also seems a little faster. I know that the down-side to this is said to be an occasional false positive, but so far there's nothing, and there never has been on the other PC.
I probably will go over to the paid version of AVG eventually, but for now I'm finding that the free one works pretty well. So Norton's bloatware approach has lost them yet another customer and saved me £25 a year, or whatever the current Norton renewal fee is.