First thing to do is a system restore on that computer. With luck you can get back to the point before the trouble happened.
Depending on the maliciousness of the file(s), though, you may be screwed worse than that.
Possibly you can get Norton to run via New Tasks and Norton can hunt down the evildoer and kill it.
If not, you may be looking at clearing the Hard Drive and reinstalling everything. That's not fun, but it will solve the problem. And -- you do have backups of all your critical systems and files, right?