I've been doing alot of Linux installations lately while figuring out how to get LinuxMCE and MythTV (Mythbuntu) installed. This is on top of pre-installed versions of Windows XP and Windows 7 RC.
One time saving tip is to know how to help Linux figure out what happened to the boot records. This is all done via a Live-CD installation and an open terminal window:
This will tell you what partition Ubuntu is, so assuming it said: (hd2,0)
The system will then tell you it found the partition and is reconfiguring everything.
Thats it. Time to reboot.
sudo reboot now
If you're still getting errors, then you'll need more than this small bag of tricks to recover. Good luck!
Update: The above instructions do not work for Grub 2.