Quit the app then relaunch it.
If you can't quit then Force quit.
Remember not to overburden a slow computer by keeping too many windows open, especially if you don't use some of them, 'cause they still take RAM.
Post again if this persists.

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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-