mac can't sleep unless parallels shut down

I run XP with Parallels 5 on an iMac. For, well, close to a year every morning I come to my machine and the Mac hasn't been able to sleep with XP running. Also, every morning when I start up I'm prompted for a snapshot. (Why couldn't this have happened overnight?!)

Does anyone have any smart ideas for how to configure Parallels or XP so that (1) my machine can go into sleep mode or (2) the snapshots are done automatically overnight (not automatically in the middle of the day when I'm on a deadline, etc.)

Thanks!