You can completely disable USB ports by using Group Policy to disable
usbstor.sys but there is no easy way to prevent users copying files over
USB. (I assume that's what you're worried about?)
XP SP2 introduced a new registry key that will allow you to make USB flash
drives read-only, but this is of limited use as it wont stop them plugging
in a USB CDRW and getting files that way, it may be of some use to you
though. This is the link:
http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/sp2otech.mspx#ECAA
Other than that, a 3rd party product may be your best bet, I have seen a
product before that offers enterprise wide control over USB ports but
can't remember what it's called I'm afraid :-(