Windows Media Player Cannot Access the File

Every time I try to open up a video in WMP, it get this error saying:

"Windows Media Player cannot access the file. The file might be in use; you
might not have access to the computer where the file is stored, or your proxy
settings might not be correct."

The video still plays in the background, but I can't hear sound. I can't
listen to my music in Itunes either. I'm pretty sure that that it's not the
first thing in the error message., and the file is stored on my computer. As
for the third one, I'm not sure.

It also says I'm not on a network, when I was yesterday.

This all started when I downloaded AIM 6.0.