Boot from a live CD--or from the Ubuntu install disk.
When Windows is installed (or sometimes repaired) it puts its own boot code in the mbr. This is why conventional wisdom is always to install Win first, then Linux.
If you boot up form the Ubuntu install disk, there should be an option to restore, repair, etc.
From a live LinuxCD (eg Knoppix) you should be able to run the grub shell (type "grub" at a terminal prompt)
Once in the grub shell, you do this:
"X" is the partition number where your grub config file is---typically this is the /boot partition of your Linux install. If Linux is on the second partition of your only drive, then you would use "1" (grub numbering starts with "0")
To find all the partitions (running from the live Linux):