Problems with two versions of Windows in one pc

Hi techyv, I need help to have two versions of Windows in one computer. My Dell system said that I needed to re-load my Windows XP Media Center to fix a problem. I reloaded but it did nothing. But now, whenever I start my computer, it loads a newly loaded version of windows, and doesn't have any of my files on it. The technician said to hold down f11 on start up and arrow down to the second version of Windows XP media center. How do I get rid of the version that doesn't have my files on it?
