Do you have any restore points you can try? Restore points could bring the virus/trojan back so be selective with the date you go back to.
My first thought would be to try and do a windows repair if you have the disk. That will replace system files in the Windows directory.
Failing that, you will need to do a full re-install I suspect. Always a hassle, but you can clean it all out with a Full install instead of battling with simple things like this. You don't know what else has been corrupted.Hope that helps