Probably there are more files missing then, but the USBSTOR files are most frequently used ones. There are copies of the files in C:WindowsSystem32DriverStoreFileRepository - just copy them to C:WindowsINF.
When XP comes with the "You must be a member of the administrators group to install. This is the case when it cannot check if the drivers are digitally signed which is true when the service 'Cryptographic Services' is not running (CryptSvc) or if its data are corrupt.
Start - Execute - services.