You mean you are getting a stop error (which is called the Blue Screen of Death)? Sorry about your blue screen but you can be absolutely 100% sure that Classic Shell does not/cannot cause a blue screen error. Something else is the cause. It can be a coincidence that you got it after you installed Classic Shell.
A blue screen error is a *operating system kernel* related error caused either by faulty hardware or drivers - not something at the shell/user mode level.
Classic Shell is just a regular application - it doesn't modify the system in ways that can cause a blue screen error. In fact it cannot cause a BSOD because whatever modifications it does are only in user mode.
Check the Event Log (System Log in particular) by starting Windows Event Viewer and use NirSoft's BlueScreenView (
http://www.nirsoft.net/utils/blue_screen_view.html) if there's a dump the OS created when it showed the blue screen. The dump will be stored at %SystemRoot%\MEMORY.DMP. Opening the dump file in BlueScreenView can show you what driver caused the error.
See if you have the latest drivers for your hardware, particularly video drivers. The graphics is the part of the system that the start menu uses more than others, so the video driver is a likely culprit.
To recover from the Blue screen, start your computer in Safe Mode (
http://winaero.com/blog/how-to-boot-int ... safe-mode/) and try the steps mentioned.