It's either located in the \Windows\System32\ (x86) or \Windows\SysWOW64\ (x64) folder, and it needs to be re-registered. Open an administrative cmd prompt, change directory (cd) to %windir%\system32 or %windir%\syswow64 (depending on x86 or x64 version of Windows 7), and run the command regsvr32 mscomctl.ocx to re-register the file.