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)?

Depending on the location, etc. it may be protected by windows, so you can’t do anything with it, that is why Eddy suggested a boot-time scan, e.g. before it becomes protected by windows or is running.