No operating system installed wipeout

This has been a persistent problem over the last several months. I have a 2010 MacBook Air and am running Win7 64bit. I don't have boot camp. When I first installed Parallels I had no problems, but now my virtual machine will crash and try to reboot, and when it does, it says no operating system installed. I copy the VM from my backup and it runs fine for a while, then crashes and burns again. I even pulled a backup from months ago, and it does the same thing. I have no problems in OS X. I run Parallels on my mini with no problems, running the same software (MS Office 2011). This has gotten really frustrating. Any ideas how to fix this before I write Parallels off as a bad idea?