Restore Grub

From Knowledge76

HOWTO: Restore GRUB (if your MBR is messed up)

Restoring GRUB (the GRand Unified Bootloader) is quite simple in Ubuntu; instead of going through all of the "gain root access" steps, and playing with confusing shell commands, you can simply use the Ubuntu installation CD to restore it without going through all of the previous hassles.

Reinstall GRUB Bootloader and MBR (Master Boot Record)

Download the Ubuntu Desktop Installation CD here.

Pop in the Ubuntu Desktop Install Disk and Boot to it.
Open a Terminal - Applications > Accessories > Terminal (One command per line)

sudo grub
root (hd0,0)
setup (hd0)
quit

Reboot and you're all done! Press escape when you see GRUB to boot into your Windows partition.