Nothing: you simply downloaded a fake file.
Be careful of .wmv files because they are often (too often) fake. Opt for .avi or .mpeg if you can.
Also, don't download stuff that looks like T_H_I_S or that show a size of around 170 KB. All fake (or maybe worse...)

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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-