Never had any problems with either avg or adaware. Occasionally run spybot S&D and all that finds is tracing cookies that adaware didn't find. Clearing cache and cookies regulary can help with this.
AVG & a weekly scan with stinger has always been ok for me, and have never really found anything nasty.
In the last 4 years i've only ever had a pc crashed by a virus once. As i make regular backups and have a full installation cd of xp pro corporate edition, i can quite happily wipe the partition and start from a raw partition, without the need to keep activating upon every install.