I installed windows 7 in the C drive, then installed Ubnutu on E, and yesterday, I installed Windows XP on drive D.
I was then unable to boot into Windows 7 or Ubuntu. I formatted drive D, but the problem still exists.
Your problem is, that the win xp bootloader is unable to load 7 / ubuntu. The problem can be fixed by reinstalling the grub loader. I don't know if you can do this without reinstalling ubuntu, maybe there is a way.