Problem: You are trying to load Windows in Parallels (Virtual Machine) and it is not loading either because you partitioned your hard drive or you downgraded/upgraded to different version of OS.
Cause: The problem is probably the boot record has been altered. Windows will not load either from loading directly pressing the ALT option or in Parallels.
Resolution: The easiest solution is, load a Windows 7 CD. It will start installation process. Go to where it says repair disk. Select the first option that says “Fix issues with the operating System”. Let it do its job. At the end of the its operation it may show a dialog, “OS X did not allow the boot record to be changed”. Click ok. Then it will say something like “The boot record in Virtual Machine was changed but it was not written to hard drive”. This will basically fix loading of Windows in Parallels.