virus not found

Hi
a Norton scan showed 5 viruses however a susequent thorough Avast scan showed none – should I be worried?
thanks

Which Norton? Norton Security Scan (the on-demand free scanner that come with GooglePack)?
Can you say what is the infected file name, where was it found (C:\windows\system32\infected-file-name.xxx)?
What avast! version and virus database are you using? (see About dialog of avast!)

Yes, you have two AVs on the same system not a good idea.

Having two resident scanners installed is not recommended as rather than provide twice the protection it can cause conflicts that could leave you more vulnerable.

Since you don’t say what was detected there is no way to even guess if there is a problem.

So examples, giving file name, location, malware name would help.

You could also check the offending/suspect file at: VirusTotal - Multi engine on-line virus scanner this would confirm if the detection is good.