1. Backup all user files and files you want to keep.
2. Get yourself an installation disc.
3. Insert disc into your cdrom tray, close cdrom tray.
4. Reboot your computer.
5. Change your bios settings to boot from cdrom.
6. Continue booting from cd and install from disc, leaving your Windows install alone.
Is that what your looking for? Cause those most likely would be the steps to migrate from one distribution to another.