The Blue Screen of Death: What do I do?

Ever since we've been providing tech support for small businesses in Toronto, we've seen countless blue screen of deaths. If you've been using PCs for a while, you've probably seen this ominous screen. You'll be working and for apparently no reason your computer monitor will go deep blue and display white error text and the only way to get back to work is by rebooting your computer. What causes this error, and what does it mean, and what should you do if you encounter one?