I suspect that the virus may be in memory: some clever viruses delete the file that they run from when they are loaded into memory and then rewrite it you before your computer shuts down.
To defeat such a virus trick, a boot time scan is necessary. This should delete the file before it has a chance to load into memory. (This doesn't work in Win 98: if you have this OS, boot into safe mode before scanning- hit F8 while booting.)
Right click the avast! globe and select Start avast! Antivirus.
avast! will do a memory scan: if it finds the worm in memory, it will prompt you to do a boot time scan: accept this and reboot.
If avast! doesn't find anything in memory, schedule a boot time scan. (Click the button at the top left of the avast! silver console and select Schedule boot time scan from the drop-down menu.)
Please ensure that you have a firewall and that your OS is up to date, otherwise infections will come back. A computer connected to the internet will be attacked by worms, viruses, Trojan horses and spyware within 40 seconds. You can prevent all of this by installing a free firewall. and updating your OS.
If you need advice on either of these subjects, please say so.
