I only add to this because it is wrong and I came across it doing a search myself.
If you have Home Premium, Professional, Ultimate and Enterprise editions of Windows 7 (but not Windows 7 N or Windows 7 KN)
To find and run Media Center you can type in the Start menu search: GoMCEDVD
If you are lazy, just type: GOMCE
It is not case sensitive so feel free to not use caps.
Then select GoMCEDVD to run the installation wizard.
Also, as mentioned if this does not work check to see if in control panel, program and features, that in turn windows features on or off, that under media features, Windows Media Center has a check mark next to it.
If you have Windows 7N edition use the above link posted by another member.
Sorry to post in an old thread, but I hate having users install software they do not need.
I checked 4 different installations here at home, 3 Home Premium, 1 Ultimate Edition and the above works fine.