USB ports on desktop computers pose a security risk in many environments. This is because simply by inserting a USB flash drive or key into these ports, users can download sensitive files from their computers and transport them off site, possibly in contravention of your corporate security policy.
In Windows XP SP2 you can prevent this by making a registry tweak that makes any USB devices that are plugged into your machine read-only so users can't tranfser files to them. To do this, go to HKLM\System\CurrentControlSet\Control and create a new key named StorageDevicePolicies. Then within this key created a REG_DWORD value named WriteProtect and set it equal to 1.
If desired you could create a .reg file for this tweak and distribute it using a logon script, or you could create a custom ADM template for Group Policy that includes this new setting.