The default for Grub2 is NOT to display the menu. However, if more than one operating system is detected as in your case, the menu should display. Some things you can try to see if you can boot xp media center. When you see the black screen after the BIOS, hit the Shift key. If that doesn't do anything you can check to see if you can boot xp. Since your grub.cfg file shows the xp media center as the seventh menuentry option, when you see the black screen, hit the down arrow on your keyboard, carefully counting 6 times which should take you to the xp entry. Hit the Enter key. I had a black screen on an install and tried this once and it worked.
The link below at the Ubuntu forums (post #2) explains in some detail the use of the Timeout options in Grub2.
At the same link, in post #4, there is another link to a page which explains in some detail the Timed display and Hidden options.