Method 1 (Drastic!):
Fill the USB ports with superglue or epoxy resin!
Method 2 (Requiring registry changes - Only for those who really know what they're doing!):
http://support.microsoft.com/kb/823732
Method 3 (Requiring you to pay for security software):
http://www.devicelock.com/
Far simpler (1):
Encrypt the data on the laptop so that copying it will be completely pointless.
or (2):
Ensure that you've got good password protection on the laptop, so that it can't be used by anyone without proper authorisation.
Chris