Mark,
The trojan has been copied to the area of you computer where automatically made backup files are kept. Windows XP makes Restore Points periodically to protect your PC, but it can also backup files which are infected.
To clear out these backup files and so remove the trojan,
1) Go to START>PROGRAMS>ACCESSORIES>SYSTEM TOOLS>SYSTEM RESTORE
2) Select System Restore Settings
3) Select Turn Off System Restore On All Drives
4) When this has been done reboot your PC.
5) Do a complete scan of your PC using your antivirus program.
6) Repeat steps 1 - 3, but deselect Turn Off System Restore On All Drives (to switch Restore Points back on.)
Hope this works.
Simon, March 2004