Do you still have your Ubuntu CD? Or any other Linux CD? Step one
will be to boot into a live desktop and backup the files that are
important to you. Then take a look at post 30 in the following thread
and see if that works for you.
ubuntuforums.org/showthread.php?t=1474253&page=3

note:
1) before trying to edit grub.cfg, back it up. "sudo cp
/vdisk/boot/grub/grub.cfg /vdisk/boot/grub/grub.cfg.bak"
2) I believe these steps are/should get you into Ubuntu from a live CD
or bootable flash drive if you have a WUBI install, which I think you
do. Otherwise, ignore the "/vdisk" part.