Had no clue where to post this, but whenever I try and stream video using WMP I get a lback screen, I hit the play buttong and nothing happens.

I use to be able to watch streaming videos online using WMP, but now I can only do so using Quicktime/Real One.

"Windows Media Player cannot play the file. If the file is located on the internet, connect to the internet"

What's the problem?

Sounds like a codec issue. BTW another good player for both audio & video streaming is VLC as it has in-built codecs & is generally very reliable. See what it can do here http://www.videolan.org/vlc/features.html
Check the link for VLC & codecs here CODECs list and alternate players for Movies

How would I go about using VLC to view streaming videos on sites that have WMP as their default player? I downloaded it a few days ago, but havent used it once.

Type in the URL address in the File menu>Open Network>Http option. Admitedly there may be the odd site that doesn't allow this but generally it's worked for me. Also, depending on your browser, there will probably be options in the preferences for helper applications. For the wmv format, you can add VLC.