for a start stick to 32 bit version, even for the XP 64 bit version which has been out for years, it has a lot of driver compatabilty probs. Atm i have come across very few probs, office 2003 works fine along with nero and all the usual progs. The only main gripe is lack of a driver for my scanner atm. If you have a prog that doesnt run as it should, just do a bit of hunting and you will find a compatable version!
As for AVG , their free version aint compatable but Norton internet security 2007 is!
When it comes to installing progs and it keeps stoping you, saying 'you dont have permision' either right click on it and 'Run as administrator' or 'click window icon,then click control panel,then user account, then, make sure you are the administrator, then, click, turn user account control off'.