I have 3 installed. Win98/Y2K/XP.
1.) I can avoid file protection by accessing system files from one O/S to another. EG: O/S Customization Hacks.
2.) I can better relate to tech support items if I can boot to the O/S in question.
3.) I can impress my friends with a multiple selection O/S screen....lol
4.) Because, I can.
Each of my O/S's access the same Favorites, Windows Addressbook & Outlook Express Store, files & folders.
EG: When I add a new Contact/Favorites/receive E-mail, the change is reflected in all O/S's.
Separate partition for each O/S and at least one other partition for misc files including the common files I just mentioned. This misc files partition is a good safety net as well. If an O/S goes south on me, it doesn't take with it any of my important common files data.
I also have a Programs Partition. This partition contains 3 folders.
Program Files98
Program FilesY2K
Program FilesXP
Each O/S partition and the Programs partition being ghosted. Misc files partition on backup CD's.
Cheers!