What OS are you using? XP? OS X?
In either case, try VLC. It can playing many file formats and it's possible that it will play more stuff than WMP can (without telling Mr Bill what you are doing).

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-