We get this query alot in regards to customers wanting to disable access to USB storage devices, without limiting the use of other USB devices. Here's a few things worth looking at:
Knowledge Base Article - http://support.microsoft.com/kb/823732/en-us (How to disable the use of USB storage devices).
Free tool - http://www.intelliadmin.com/blog/2007/01/disable-usb-flash-drives.html (two versions: local and remote).
Paid solution - http://www.lumension.com/usb_security.jsp?rpLangCode=1&rpMenuId=118468 (lots of options).
There are many possibilities (e.g. disable writing only), check http://search.live.com/results.aspx?q=disable+usb+storage+windows+xp&go=Hledat&mkt=cs-cz&scope=&FORM=LIVSOP.