I remember a similar problem getting rid of a virus/trojan, found out it kept coming back because I has system restore on. Had to turn restore off, clean the system then re-enable restore. The virus was sitting in the restore file and would just come back every time I booted up. That solved my problem. Maybe you need to go that route too.
This from Symantec regarding
JS/Downloader
Discovered: October 27, 2003
Updated: February 13, 2007 12:12:27 PM
Type: Trojan Horse
Systems Affected: Windows 2000, Windows 95, Windows 98, Windows Me, Windows NT, Windows Server 2003, Windows XP
The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
- Disable System Restore (Windows Me/XP).
- Update the virus definitions.
- Run a full system scan and delete all the files detected as JS.Downloader.Trojan.