I'm using a Windows XP and tried to open the Device Manager today. To my surprise, I got a "MMC could not create the snap-in" error screen instead (pic):
I recalled that 2 weeks ago I encountered the exact same problem after I ran a third party 'Registry Cleaner' software to fix some device problems my pc was having. That time I solved the problem by re-installing Windows XP.
This time I thought well there must be an easier way to fix the problem. So I searched the Microsoft support site and see if I can find a solution related to the problem. Thanked my lucky stars there was a solution for it. The solution is in this Microsoft support page. It suggested downloading a hotfix to fix the problem. So I downloaded the hotfix and applied it. It worked swell. My Windows XP device manager is working again. No more "MMC could not create the snap-in. " screen. What a relief that was.