I've been using AVGfree for awhile now on my personal laptop and haven't had any major issues in general (i.e., not Alibre-related, I just recently put AD Express on my laptop). The only hiccup I have with AVG is every now and then it'll fail to connect with the automatic update server, so I have to go in and start the update manually. Still not a big deal though.
We had purchased versions of Norton on all our home computers, what a disaster that was. Not a happy camper with them at all; messed with a lot of our files, WAY too complicated to use, brought our cable internet down to a crawl, and right after New Year's my laptop still got hit out of nowhere with a Trojan virus.
After getting my machine totally rebuilt, a friend of mine told me about a program called NOD32 by Eset. He swore by it, and when I first got the 30-day free trial I really liked the way it worked. But all of a sudden it stopped updating and kept asking me for a license code (this was maybe a week and a half after starting my trial). I dunno, maybe I'm jumping to conclusions, but I decided to toss NOD32 and put AVGfree put back on. It is free after all.
My computer-hardware nutcase brother just got done putting together his first fully-custom rig, I wanna say he's got AVGfree on it as well but I'm not sure, I'll hafta ask him.
My tower at RIVCO has PCTools' Spyware Doctor, dunno how much it costs but it seems to do the job pretty well. I have it set to update when I log-in in the morning and that seems to happen almost instantly, and it runs a quick scan around 3:00 just before I leave for the day. I do notice the system slows down a bit while the scan is running (especially Alibre), but not to the point where I have to stop working. Just try not to do too many complex things at the same time and it's fine.