Kind of hard to determine, try Safe mode and Repair disk etc
Perhaps you can get away with just reinstalling OS X and that will fix it
Instructions on this link, including Blue Screen and what your options are etc.
So how did you manage to get around this problem? Re-installed Snow Leopard? I encountered the same problem minutes back with a blue screen. Cant click Apple icon. Shortcuts dont work. No file opens - returns error code 10813. I can get in and out of a folder though - but nothing much. Cant copy paste as well.
MacBook Pro'13 2009 model
Mac OSX 10.6