What you did wrong was to delete the partition with Ubuntu on it.
Ubuntu installs a boot loader called GRUB (it's the bit that gave you the option to run Ubuntu or Windows). GRUB is installed in the Ubuntu partition, and Ubuntu modifies the disc's master boot record (MBR) to use GRUB rather than the default Windows loader.
To fix it you can figure out how to reset the machine to it's original (out of the box) state (and for that you will need the manual and the original hidden partition that comes on most laptops these days), or you can reinstall XP from scratch, but you need and XP disc for that, or you can borrow and XP disc and follow the instructions on
http://www.ehow.com/h...loader-from-dual.html