How to fix the boot loader with an installation disk at hand

I recently messed up my Master Boot Record and my system is not booting. I was dual booting Windows and Ubuntu. Both of them are not loading now. I don't care if I lose my windows, but I want my Ubuntu back!!

How to get it (I have a fresh Ubuntu 10.10 LTS disk with me). I went through the install wizard but feared that I may loose my existing installation/setup. Thats why I came here to check what is the ideal procedure to only update the Boot record instead of a full installation.

This is covered in detail in the Ubuntu Community Documentation.