First things first, I'd run a virus scanner since that file is a common target for virii.
If that doesn't work (you don't have one and can't install one), I'd (if possible) plug the drive into another machine and scan it.
Now either way, virus or not, you're going to have to replace that file, which can be done by inserting the XP disk and booting from it, then selecting (repair installation). If you're unable to scan for virii before you do this, I'd install a scanner before doing anything else to prevent this from happening again.
If you're savvy enough I'd just copy the new installer dll's over...but it might actually be a bit easier to just follow the method above.
good luck!
