This is a copy/pasted guide and not mine so please read carefully and ask any questions if you are unclear on what to do.
Make sure your Windows CD-ROM's in the drive.
Go to Start/run, and type SFC.
Choose 'Extract One File From Installation Disk'.
Type rundll32.exe, not worrying about its location. Then, click Start.
Next to 'Restore From', type in or browse for the file’s location, which is probably in the WinME folder of your installation CD-ROM (typically D:\WinME), or in your Windows\Options\Cabs folder, as the case may be.
Then, next to 'Save File In', enter, or browse to C:\Windows, and click OK. System File Checker looks for the file, saves it as you requested, and then tells you that 'the file has been successfully extracted'.
That should put things right.