I think that ABers will need a few more details before being able to answer your question (but see below). It would help to know the following:
1. Which operating system are you using? (e.g. Windows ME or XP?).
2. Are you using dial-up or broadband?
3. When you say that you can connect via another user's account, does this mean that your own PC will connect using the other account details or do you have to use the other person's PC?.
Having asked those questions, however, there is one thing which you can try anyway: System Restore. If you simply restore your PC to a date before the suspect download, it's quite likely that the problem might disappear without having to ascertain its exact cause.
Chris