You should tell us, if you can, what the virus is called and what file is infected.
Otherwise, normally, when you have XP, you should disable System Restore (you will lose all restore points), restart into Safe Mode, and use your AV to scan and remove virus.
Some viruses write entries to the registry and create Run or Run Once entries on startup, and you need to terminate the virus process in Task Manager before you start removal. That's why it would help if you can tell us what the virus is. Or give us the link to the download page for the removal tool which you have downloaded, so that we can tell you what else you can do.