turned my comp on this morning and when it all loaded up onto windows(xp) i got a little message come up from norton antivirus saying that its dectected trojan horse virus!!

it says unable to reapair file so i click on ok and it then says access to file is denied and this message will not go off my screen!?

also i went into the location that the file is(it tells me on this message thing) and then i tryed to delete it but it says it cant as the file is being used by someone else!

i thought i had a top notch anti virus and i got windows firewall turned on and all up to date, like whenever it prompts me to update i always do, so what do i do now cos this file will not go, my computer is running fine at the moment though?

HELP!!!