I plugged a USB mouse into my XP Pro SP2 desktop PC.
The Windows mouse drivers it uses are mouclass.sys and mouhid.sys, both in the Windows\system32\drivers directory.
Check that these are present - if not they should be in the Windows\Driver Cache\i386 cab files.
All the .inf files are in the Windows\inf folder.
You may be able to force the installation of the drivers in device manager - select the unknown device and manually enter the path to the location of the .inf file - either the .inf folder above for the default MS drivers, or the drivers that came with your mouse.
SD