OK. FAT32 or NTFS doesn't matter. This is not the cause of your problem. The file system has absolutely no effect on video cards.
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Did you try a different copy of the Aquarium?? (earlier version)
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Do you have a spare Hard drive around??
format it.
put XP on it
install video drivers
try Aquarium screen saver.
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Can you try an openGL saver from
www.reallyslick.com
do you have the same problem with these??
I am racking my brain over here.
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try different refresh rates????
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Last but not least; it could be the card itself. Can you RMA it??
Don't lose faith in ATI, I think they are just as good as nVidia.
(personal opinion, but hey..) :P