I changed from AVG free to AVG 8.0 on two XP machines (thanks for the very useful info here).
The kids desktop seems to be just as usual, overburdened with security and games, and all the better for more security.
The laptop is a fairly reasonable spec, not blistering but adequate.
I only use this for running the BBC IPlayer through a big monitor and nothing else.
It seems that running a big monitor with the IPlayer is already pushing it pretty close to 100% of its capability, and switching to AVG 8 pushed it over the egde, because it ended up as a 5 second refresh set of stills instead of video.
I removed AVG 8.0 and it went back to normal again. I haven't put any antivirus back on it yet, but for some reason AVG free edition does seem to be a bit lighter on its toes than 8.0. Purely subjective though.