Best way is to boot the ubuntu CD. On the CD is an option for "Rescue Mode" (not graphical rescue mode!!) under Advanced Options. Go through those steps, and then choose to reinstall GRUB.
1. Boot your computer up with Ubuntu CD
2. Open a terminal window or switch to a tty.
3. Type sudo grub. Should get text of which last line is grub>
4. Type "find /boot/grub/stage1". You'll get a response like "(hd0,1)".
Use whatever your computer spits out for the following lines.
5. Type "root (hd0,1)", or whatever your hard disk + boot partition
numbers are for Ubuntu.
6. Type "setup (hd0)", to install GRUB to MBR, or "setup (hd0,1)" or
whatever your hard disk + partition # is, to install GRUB to a
7. Quit grub by typing "quit".
8. Reboot and remove the bootable CD.
Try method 1 first!!! I got this from the ubuntu forums. I advise you to ask your question there. DONT JUST TAKE MY WORD FOR IT.