Use a system restore point to go back to a working state (if that function is activated in your XP). A lot more involved (and gratifying when successful) method is the following:
Retrieve the file version from the .dll and check on the internet if that dll is part of a service pack, windows install CD or KB fix. Looking at the name I assume it will be a part of the installation CD.
Then use 7-zip, WinRAR, WinZip, or any decent file manager to search for the dll in the .cab files on the CD. Extract it and copy it to the system32 folder.
If you are (too) trusting (of) the interweb, you could also google and download it directly.