If you ever get the dreaded "Missing or Corrupt Windows System32" error message then this set of instructions from the Microsoft site can be of great help in getting your registry and other system files restored.
The gist of the instructions is to show how to get your XP system to minimally boot so that you can manually get to one of the restore points.
I tried it and the instructions generally work. However, if you run into a lot of access denied problems then you may need to reset the file permissions so that you are the new owner. You can do this by right-clicking on the file or folder and then selecting the security tab to add the new owner name.