1. It isn't advisable to have two resident AVs installed on the same system, they can cause conflict and lock you system or leave you more vulnerable rather than less. What was the other AV and what was detected and where was it found ?
2. No one AV will cover every virus, trojan, adware, spyware or malware infection, there are specialist tools for different areas, see below. avast! would just become too weildy if it were to try to detect absolutly every piece of adware, spyware or malware, a specialist in one area is always going to be better that a jack of all trades, master of none aplication (IMHO).
avast is a specialist anti-virus program many trojans are spyware related and whilst avast does detect some of them it does't detect against everything.
If you haven't already got this software (freeware), download, install, update and run it.
1.
Ad-Aware2.
Spybot Search and Destroy3.
Spywareblaster Don't install this until you are clean.
4.
Ewido Security Suite If using winXP. or
a-Squared free if using win98/ME.
If you are not getting a virus warning that you believe is a new, undetected virus, then if you can zip and password protect ('virus', will do) the suspect file and send it to virus @ avast.com (no spaces), or send from the chest.
Give a brief outline of the problem (possibly a link to this thread), the fact that you believe it to be a either a new, undetected virus and include the password in the body of the email. Some info on the avast version and VPS number (see about avast {right click avast icon}) will also help.
You could also check the offending/suspect file at:
Jotti - Multi engine on-line virus scanner if any other scanners here detect them it is less likely to be a false positive. You can't do this with the file in the chest, you will need to move it out.