Becca
this is the exact same problem I've encountered.
I downloaded an AVI file I cannot delete. I've been on it for hours over a few days. It is really p***ing me off as my computer is normally very fast but now it practically ceases up and constantly says it running at 100% CPU capacity even though nothing is running. I presume this file is causing the pc to slow down as it must have a virus or worm on it.

I've deleted Limewire (I'm now extremely sceptical about it). I run on Windows XP. I went onto my support line who said Limewire was a dirty word. They told me to look on the faq on Limewire.com about how to delete it from the seeding. Though they wouldn't tell me how to do it (it was 15 mins before they were going home). They also said try deleting Limewire and then delete the file.

This is the news I've been waiting for. I am at work at the moment but will try both the links above as soon as get home and hopefully get rid of the damn file for good. Hopefully that will speed up the computer again.
Cheers