I'll admit, I fell right into this one.
My Win 7 VMs kept suspending while I was off working on another machine. I'd come back to the host machine and see VMWare Workstation idling with the Win 7 instance frozen in suspend.
The default power configuration for Win 7 -- even when on the machine is "plugged in" -- includes dropping into sleep mode after half an hour.
This, thanks to VMWare, translates into a "Suspend Guest" operation.
Maybe Vista shipped a default power configuration like this. I don't remember it being the case, but, to paraphrase Pulp Fiction, "[Vista] may [save power] like pumpkin pie, but I'd never know 'cause I wouldn't [run] the filthy mother****"