Normally the ubuntu installer tries to detect your Windows installation.
You can probably still boot both linux and windows. You will have to pass some commands.
Can you get a grub prompt?
Then type this:
title Windows Vista
The hd0,0 says windows is on your first hdd and the first partition.
The title does not matter.
Otherwhise you will have to bootup your system again using the Ubuntu live cd.
If your harddrive is /dev/sda, then you need to do
Please let us know from the moment you can get in an operating system. Be clear in what is happening.
To make these changes permanent you should first be able to boot into linux succesfully.