Open the Windows Registry and open the following key
HKEY_LOCAL_MACHINE\System\CurrentControlSet\ Control\StorageDevicePolicies

Now add a new DWORD called WriteProtect and put the value as 0 to disable write privileges to the USB port. To reverse the step, either delete the WriteProtect REG_DWORD or toggle the value to 1 which will enable the port.

Remember that the above trick works only with Windows XP SP2.

If you like to go a step further and disable users from connecting USB storage devices to their computers, here's the trick:

Open registry and navigate to the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet \Services\UsbStor

Now in the right pane, double-click Start and type 4 in the Value data box (Hexadecimal) and quite the registry editor. To enable the USB storage devices, change the Start value back to 3.