Mine used to do exactly the same, until one day when I wasn't spending even more money on cars I bought Kaspersky, haven't looked back since, found out though I had remnants of all the old free debug programs on it still that were clashing with each other.
Kasper is a little slow to start with as it learns what it can trust and what it can't, but ace after that, but do notice it depends on time of day, slower noticeably when more people on the net.
Now a quick scan on me lappy(also kaspersky) takes 7 seconds, mine about 1 minute, he's on Vista, I'm on XP, but soon going Vista hopefully.
tabetha