What about dual or multi-boot? Just install to another partition, perhaps another partition on another hard drive. Note #1 - assuming it's the same as Windows 7, if you don't want to end up with Windows 8 using different lettering for you partitions, start the installation while running an existing version of windows. Note #2 - if you're installing a 64 bit version, the installer is a 64 bit app, so you need to be running on a 64 bit version of windows to run the Window 8 64 bit installer (at least this is how it works with Windows 7). This can be XP or Vista or Win 7 64 bit.
I had to do this to add Win 7 64 bit to an existing dual boot XP 32 bit / XP 64 bit system. I started the install process while running XP 64 bit. If I had started the install by booting from the DVD, it would have re-lettered the install partition "C", and removed the letter from the original "C" partition. Starting the install from XP 64 bit, left my drive letters alone (in my case, I have 3 hard drives, 5 partitions each, I use "D" for XP 64 bit, "I" for XP 32 bit, and "N" for Win 7 64 bit, "C" only has BOOTMGR and NTLDR and related loader typs files.
What is your currrent OS?