Just as Microsoft had disabled Games by default in the Business and Enterprise editions of Windows Vista, bowing down to pressure from Employers and the Corporates, games have been disabled in Windows 7 Professional Edition too!
Apparently Minesweeper and games in general have been responsible for millions of hours of lost productivity !
Enable Games in Windows 7 Pro
To enable these missing games in Windows 7 Professional edition or Windows Vista Business and Enterprise editions, open the Control Panel > Programs and Features > LHS pane, click Turn Windows Features on or off > in the Window that pops up, check the games you want or click all.
You will not be able to do this in Windows 8. You wont see the Games Explorer too now. Microsoft may have decided to do away with it, since the focus is now on Games as Apps.