Someone posted a link to a ubuntu install that automatically configured itself to dual boot with windows. Currently using it now and sacked off Windows (but the missus still uses it so I cant scrap it).

I can't remember the thread it was in – don't suppose anyone remembers the site it was on?

http://www.singletrackworld.com/forum/topic/two-oss-on-a-pc-doable

or direct

http://www.ubuntu.com/GetUbuntu/download

I had a Vista PC (Lenovo N500 from work) that I wanted to keep Vista on for testing purposes and all because there's no Vista media, but I also wanted Ubuntu. The following doc helped as it tells you how to use Vista to create some (variable depending on disk size) space on the disk and then install Ubuntu. It then uses the GRUB boot loader to allow you to choose Ubuntu or Vista. Worked a treat for me.

http://apcmag.com/how_to_dualboot_vista_with_linux_vista_installed_first.htm

If Windows is already installed, and when you install Ubuntu, you decide you want to keep it, Ubuntu will set up a GRUB bootloader which will give you the options to boot to Ubuntu or Windows, with Ubuntu as the default. I believe you can modify the GRUB file to make Windows the default, but I never did.
Also, IIRC, Ubuntu will tell you how much data is on the Windows partition and will allow you to shrink it – remember to leave enough space for future use.
I'm pretty confortable with all this but..

BACK EVERYTHING UP FIRST