Extract a fresh copy from your install disk or the cabs on the hard drive if you have them. I do not know what Operating System you are using. If Win95, DOS extract or Winzip will do it. IF 98 use System File Extractor. In Win Me there's anothe method. Here's a link with instructions for each OS.
Main.cpl belongs in C:\windows\system
http://support.microsoft.com/support/kb/articles/Q129/6/05.asp
Main.cpl belongs in C:\windows\system
http://support.microsoft.com/support/kb/articles/Q129/6/05.asp