Phil,
Sounds like you've fried your Master Boot Record, due to the system restore GRUB (the boot loader) cannot load from the correct partition. Check out this post over on the Ubuntu Forums for instructions on restoring GRUB from the Ubuntu Live CD,
http://ubuntuforums.org/showthread.php?t=224351
Other than that, you doing a system restore may have wiped your Ubuntu partition. If that happened just reinstall Ubuntu again or overwrite the current MBR by using the an XP CD and repairing the install, that will get you back into Windows.