I would suggest that you try a repair of avast. Add Remove programs, avast! Anti-Virus, Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.

If that doesn’t work, uninstall using add remove programs and the uninstall utility, reboot, install.

Was it avast that reported to trojan or some other program? There have been a number of reported false positive problems with spy sweeper and when action was taken in spy sweeper, it screwed avast.

If avast reported it (which I find hard to believe) answer the avast related questions as well.
- What OS are you using? is it up to date?
- 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)?
- What actions have you taken to try and resolve the problem?