suggestion test
run windows search. Search for "Mouse.drv". it is possible that you have had a virus that has imaged, and locked your system folder, and copied it's contents. If this is so, then there is nothing you can do but run the Windows recovery disk and do a repair install. I don't know any way around it apart from using third party "fix it" software like TuneUp2006, but I dont know at all if that would fix it.
If you find Mouse.drv, have a look at the location. Then navigate to that location, you should find all the other system files there too.
NOTE
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I'm at home now, and don't have a server system, so I'm just guessing that the mouse.drv file is in the 2003 system folder as it is in the XP file system. If it's not, either find out a file that should be in any server 2003 c:\windows\system folder, or I will let you know tomorrow morning when i get back to the office. Try the Mouse.drv, it should be the same.
Hope this Helps.
Neil J Cotton
njc Information Systems
Systems Consultant