Run every possible scan application you have:
Disk Utilities' First Aid, Antiviruses (most unlikely that it's a virus but you never know), anything.
First, of course, get the latest security updates with Software Update.
Also, try dlding a .dmg file from a website (less chances to be corrupted)

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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-