Still, I wouldn't erase my Windows partition if I were you, it can be handy sometimes. Plus, if you erase it, you basically lose your Windows license (and if you paid for it, well, common sense says to keep it). Especially if you have a large drive.
You can install a lots of operating system on a single drive, especially if you install Linux-based OS, since they can be installed in extended partitions (something Windows OS don't tolerate IIRC). Don't mind if you install it after or before installing Windows; the boot loader coming with Ubuntu will find your Windows OS (simplification) and will actually let you choose which operating system you want to boot when you start your computer. Try creating a new partition on your disk (you can do this with tools coming with the live CD, like gparted or cfdisk, preferably after defragmenting the partition you will split) and install your new OS there (you'll also need another partition for the swap partition).
Here's a link that might interest you, talking about how to install a large number of OS on the same system:
http://www.justlinux.com/forum/showt...hreadid=143973