as it seems u have just one partition of c drive....... so go like this.
download a partitioning tool, like Noton parttition magic portable, just 9 mb download....use it to make two partition out of your c drive.....
1 512 MB swap partition
2. Root partition (ext3 format) of size around 6 GB (will be sufficient as you will be using your windows partition for documents).
Now install Ubuntu on this root partition.
To not use windows boot manager, you have to do this:
while installing ubuntu, when the installer asks questions like name and all, in the last page when it displays final information, there is a button labelled advanced, click that, it will show (hd0) as location of installation of ubuntu boot loader. change it to (hd0,2).
[ Expalation you root drive is like /dev/sda3, which translates to (hd0,2) in terms of linux language)
Now boot into windows, use a software Bootpart to add the location of ubuntu boot loader to windows boot loader.
This way your windows boot manager will remain intact. And you can remove Linux boot loader location entry from it anytime, by simply editing c:\boot.ini file......
I haven't explained stepwise but i hope you can do it now. give it a try....if you don't get anything, ask again here......