A. Stop hacking! I've seen this when I was experimenting and tried to modify the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\ProductOptions\ProductSuite value to test a program I was writing and it just crashed the OS!
I think it could have just displayed a warning but Microsoft decided to teach us a lesson :-)
If you are not changing keys and still get this crash, check any programs you might have running which may be trying to modify keys that it should not!