Well to realize what you want to do, you will have to have a partion solely dedicated to Linux. Now there is a program called Norton Commander which will give you the option at startup to boot to the operation system of your choice, a bit like XP does when it gives you 30 seconds or so to choose your operating system.
Norton Commander is not a new program, but if you search zdnet or cnet you may find equivalent types of programs (some trialware will do).
Once it is installed, you will have the option to install you new operating system to your designated area on your computer.