In order to help fully we need more information....
- What avast! version and VPS file (virus database) number, e.g. 0436-4 (see about avast!)
- What was the virus name, what was the filename, where was it found
example (C:\windows\system32\infected-filename.xxx)?
- Do you have a firewall, if so what?
Download and run this program,
Ewido Security Suite it specializes in trojan removal and may well be able to kill this.
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.