Your post doesn't narrow it down much.
I have a new Dell Inspiron Media version of Win XP- a Laptop I am using as a desktop replacement. It simply won't remain on Standby for longer than 10 or 12 minutes and then goes back to desktop and then in time, the screensaver.
Under "BIOS settings Effecting USB" I'd take the advice on just one item.
"Power Management ? Disabled
* The Operating System should control Power Management."
Don't make any other changes notes by this article at http://www.usbman.com/Troubleshooter%20General.htm
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