Hi Agapanthus,
Two resident antivirus programs is one too many. Non-resident scanning programs work fine alongside each other, lest their signature files don't "fight"each other (e.g. avast and panda-online are no good team, but that is panda's fault).
Most on-demand scanners can be a good companion for a resident AV solution like Avast. I had good experiences with Bitdefender, SSM )intrusion detection), a-squared anti-malware, ClamWin anti-virus scanner, DrWebCureIt, stinger-exe, F-Prot for Dos with the automatic uploader, DrWeb hyperlink scanner plug-in all worked brilliantly alongside Avast.
polonus