I recently had a bad infection of malware/spyware/virus and tried many things to clear it and downloaded a few new progs to try as part of the whole thing.
One of these that seemed good was Avast - it found what nothing else found - and so I'm still running it. But I've always been running AVG, too and I was browsing these forums and saw a post where someone claimed you should only ever have only ONE anti virus prog.
Is this right would you say?
I've currently got Comodo firewall, AVG and Avast, Superantispyware and Peerguardian running from startup.
In one word NO. The ABSOLUTE rule is only one anti-virus program running and one firewall. Now that is not to say that you cannot have two of them installed, though I personally wouldn't recommend that either since very often once you get an anti-virus program installed it is very hard to keep it from running.
The reason behind this is that often times they will end up battling each other, slow the computer AND often times let something onto the computer that shouldn't be there.
If you want to use more than one virus program to be certain there isn't anything the onboard program is missing then most will suggest also using an online anti-virus scanner, ESET Scanner for instance, is a very good one and highly recommended.
PhilliePhan has some good suggestions for online scanners in his sticky HERE