Update your operating system and web browser (which would be handy to know).
In order to help fully we need more information....
- 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)?
Visit -
http://www.avast.com/eng/avast_cleaner.html read the information on the page - download and run the cleaner.