You don't say what you did when avast detected it or why avast couldn't deal with it ?
I suspect because it was in one of your windows system folders and as such protected by windows.
What Tech suggests in step 1 & 3 should be able to deal with this but it would be wise to complete all steps.
Deletion isn't really a good first option (you have none left), 'first do no harm' don't delete, send virus to the chest and investigate. So when you next encounter anything think before geting rid (deletion) of a file.
You might also consider proactive protection, in order to place files in the system folders and create registry entries you need permission. Prevention is much better and theoretically easier than cure.
Whilst browsing or collecting email, etc. if you get infected then the malware by default inherits the same permissions that you have for your user account. So if the user account has administrator rights, the malware has administrator rights and can reap havoc. With limited rights the malware can't put files in the system folders, create registry entries, etc. This greatly reduces the potential harm that can be done by an undetected or first day virus, etc.
Check out the link to DropMyRights (in my signature below) - Browsing the Web and Reading E-mail Safely as an Administrator. This obviously applies to those NT based OSes that have administrator settings, winNT, win2k, winXP.
