Windows 7 problem

Hello,
I just built a new computer using Windows 7 and everything was fine for a couple of days. I started loading the computer with files. All of a sudden my screen starts showing Windows Media Center and no matter what program I try to load, the window media center appears.
If I close it, it just pops back again.
After several attempts to close it I get my windows screen back, but if I click on anything, media center comes back.
Got some ideas???