Where is uistub.exe?

What is the usual path to uistub.exe (for Windows XP Media Center Edition Professional)? Is it in Windows? Windows/System? Where? When I downloaded NAV 2010, it automated things I’d previously had control over, like updates. The only way I could stop it from constantly updating in the background was to disable uistub.exe by renaming it. Problem is, I’ve forgotten where the file is and the latest (manually run) updates are searching for it.