We will need more information to be able to help you:
- Which OS are you using? Is it up to date?
- What avast! version?
- Do you use a firewall? Which one?
- Do you have any other antivirus installed in your system? Did you have in the past?
- Any other security programs that could interfere?
Do you have any rootkit tool to scan your computer?
If you haven’t already got this software (freeware), download, install, update and run it, preferably in safe mode. AVG anti-spyware or SUPERantispyware or Spyware Terminator.
Better, SuperAntispyware.