I have boot-scaned 3 times, and still about every 20 min to hour i will get a warning again about the virus.
I assume that this is a detection by the web shield provider, giving a location in the internet ?
If not what was the file name, where was it found example (C:\windows\system32\infected-file-name.xxx) ?
So it looks like you may have a trojan on your system that is trying to download more malware and that is being detected by avast but not the downloader trojan, as Spiritsongs said Ewido for XP another if win98/ME, etc. is a-squared.
If you haven't already got this software (freeware), download, install, update and run it, preferably in safe mode.
Ewido Security Suite If using winXP. or
a-Squared free if using win98/ME.
Do you have a firewall, because that should be able to stop unauthorised outbound connections ?