I use Avast on my secondary machines; the machines that are just for gaming or testing. The updates are not noticeably slow and I have never had a virus on them either (I check the drives with Kaspersky through the network once a month) so I cannot speak for its disinfection rate. I do know it has a larger TSR and CPU footprint than others (about the same as Norton) and it is sluggish when scanning (slice hog).

SPECS: 4 TSR Loads (1,924K; 14,188K; 34,428K; 208K == 50,748K)
Slice Time: 10M13s of 86H up time @ No system scans, 1 startup (Critical system) scan.
Time Slice == 5.10 @ 1 startup scan (no system, daily scans). Time slice is figured on the number of seconds used by uptime of system.

Even with these limitations (?) I would strongly suggest you get this, or another product like it, if you have no virus protection at all.

Yeah, this is what I use, gonna grab kaspersky at some point, but avast has been fine for ages. I've never had an issue with getting infected, only a handful of times I've ever downloaded something that got flagged (notably, Daemon tools always seems to get picked up because of the toolbar or whatever packaged with it)

I used Avast since they were the FIRST to come out with a 64 software for my machine.
I'm running it for 4 years both 32 and 64 machines and works excellent. Updates "smart" (as you enter some new door let's say into p2p (torrent) or sop immediately updates). Infected mail never passed. Boot time scan its hes best as windows won't load and any file is a "sitting duck". The web shield just advertise you (in real time) to abort connection on some dangerous web pages. I recommended it and installed on some friends machines (repacing Panda and AVG)and its doing (much) better everywhere. Of course Avast isn't God so its up to you to controll your (PC) behaviour. Firewall (even windows') its a must and highly recommended an antispyware like Seek&Destroy(with TeaTimer) to work together in your favour.