can I run two anti-virus programs at the same time?

Factual Questions
1

I just got invaded by a virus that is causing me real problems. So in addition to running the AVAST antivirus (free version) which I’ve been using for 6 months, I downloaded AVG antivirus. (from http://free.grisoft.com)

Then somebody told me “what, don’t you know? You’ve committed a mortal sin that offends the viral gods! Never, never, never have two antivirus programs installed and running on your computer” ( a standard PC, Windows XP sp2, with 1000k of Ram)

So, geek dopers, please fight my ignorance: what’s the straightdope on this one?
Are 2 heads worse than one? I just assumed that if one antivirus program missed something, maybe the other one would catch it. But do they interfere with each other? Am I now doomed, or is there yet a chance to confess and redeem my sin?

2

The problem is that antivirus programs need to be able to access your machine at a pretty low level, and most of them aren’t prepared to share that access nicely.

3

You can and I have (Norton and AVG) for months. I no longer do though.

I had no problems. However, you run the risks of having problems.

4

If dumped one, which one stayed?

5

No i would only use one. I personal use AVG, and Zone alarm firewall.