Run a duel booting system. This means that you can have two OS on one pc. When you boot up the pc you will get a menu and you can choose which os you would like to load.
To do it I would use mepis 3.4-3 (linux distro) which is a live and installation disk. When you start you pc in the bios say you would like to boot from disk. Once loaded and looking at the desktop run QT parted which can be found somewhere on the main menu. Using it create an ext3 partion leaving the windows partion alone, then save the changes. Go back to the desktop and click install me. Follow thorugh the installation process.
Remember to back up before hand.