Prior to Win8 I had XP, Vista and Win7 all on the same HD plus a storage partition. When Win8 came out I changed this to having only Win7 plus XP virtual on one partition, plus one storage partition. And installed Vista by itself on a separate computer. When I installed Win8 on a third primary partition on top of Win7/XPV, I kept getting BSODs and Win7 appeared to be corrupted. Then when I removed Win8, Win7 would not boot and the installation disk could not repair the start menu. Had to reload in the end. Since then installed Win8 on a separate HD with normal HD disconnected during installation. I now have both HDs running on the one machine, but there's no connected boot menu, and can select which one to boot from BIOS at startup.
If I eventually decide to use Win8 as my primary OS, will put Win7 on separate machine, and probably ditch Vista. I have kept Vista until now, as I felt I had not got my money's worth out of it, and there were a few features I wanted to keep. For example, in Windows Mail in Vista, you can format the background with sound, color or picture, which you can't do in WinLiveMail. I only use XP virtual for legacy games.
Update:
Discovered I could hot connect Win8 West Digital Caviar on external SATA2 port. If Win8 HD was connected prior to boot it would always boot to Win8, without any boot options to 7 showing. If Win8 HD not connected would boot to 7 and then connect Win8 HD to SATAe port, no problem.
Then for no apparent reason 7 became buggy. First XP Virtual wouldn't boot. Then 7 became corrupted. And finally 7 wouldn't boot and wouldn't repair boot menu off installation disk.
That's it for me. Enough is enough.
I will only run 8 on separate machine by itself. And that's OK since when the RTM is available plan to run it as upgrade on XP Pro retail as clean install on other machine anyway.