Virus c:\winnt\explorer\explorer.exe

On my 900 Athlon, Win XP, i recently received the following error message (via Norton Antivirus 2002):

Norton has detected a virus on your computer.

Object name: c:\winnt\explorer\explorer.exe
Virus name: Backdoor.Trojan
Action taken: Unable to repair this file.

My question is, with the importance of this file in mind, what’s the best course of action in fixing this? If I delete it (or move it), what’s the best way to reinstall JUST that file (which is an integral part of the operating system).

Thanks in advance for any suggestions.

Joe