Well there could either by another element downloading the malware again (you have to find that) or you keep visiting the same site that infected the system to start with. Or it could be being re-infected via your network, other systems not clean.
What is your firewall ?
You might also consider proactive protection (if this is possible on your network), 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.
If you haven't already got this software (freeware) to try and find any other elements of this infection, download, install, update and run it, preferably in safe mode.
1.
AVG anti-spyware (formerly Ewido) If using winXP. or
a-Squared free if using win98/ME.
