This question already has an answer here:
- How do I install Ubuntu? 4 answers
Can I install ubuntu OS over the Vista OS on a Compaq laptop. Are there any instructions out there for this???
Check out the WUBI installer: http://www.ubuntu.com/download/desktop/windows-installer
This gives you the impression that you are installing Ubuntu 'over' your existing OS. In reality, it's just a nifty tool that does replaces the legwork need to go the old fashioned route of having a 'dual-boot' setup in which you have a separate disk partition (or physical disk) for each OS you want. There is a selection screen (called a bootloader) after the BIOS at which point you can select which OS you'd like to boot.
The only way to truly have one OS 'over' another OS is to use virtualization of some sort. There are many options here. If you want to go that route, it would be best to do some research and come back here with questions :)