I can't seem to get GC2:TA running (and it's the only GC I wanna play at this point )
I figure it's not SD's fault I'm beta testing a new OS, and I'm going back to playing Xcom for a while...
It works perfectly for me in Windows 7 x64, so I don't know why it won't run for you. Not tried DA or DL though, I only play TA these days.
As for losing partitions, on my computer I experienced that Win 7 didn't assign a drive letter to my XP harddrive after starting up (separate drive though, not just partition), but that was easily sorted with the disk manager in the administrative tools. Multi-booting worked perfectly too; I have no problems booting XP.
I've not tried a whole lot of software yet, but so far everything I've tried that I had in XP works in Win 7, although with some odd quirks sometimes (like Impulse reporting the missing Flash.ocx or something like that)