There's 2 versions of USB... USB 1.0 and USB 2.0 (aka High Speed USB).
Version 1 is 12 megabit per second.
Version 2 is 480 megabit per second.
Version 2 has been standard on most computers for the last 3-4 years, but if your computer is older you might have a mix of v1 and v2 ports, or only v1 ports.
Also, WinXP has USB 2.0 driver support bundled into one of the service packs... I want to say SP2, but I can't remember. The native OS downgrades USB 2 to USB 1 speeds, which sucks. Make sure you have SP2 installed on your computer.
Transferring files at 12 megabits per second sucks though. If you find your thumbdrive is painfully slow, look at your USB speeds for a potential solution.