In an excellent article at Slate, Paul Boutin explains why your computer crashes:
A few years ago, Microsoft set up the Windows Error Reporting Service to help find out where crashes come from. After a Windows application — or your whole PC — shuts down, a box pops up asking you to send a confidential error report. Using pattern-matching software to sift through the data from millions of these reports, Microsoft discovered a surprising statistic. Seventy percent of Windows crashes involve one particular kind of software: device drivers.
Yep. And I would guess that 70% of the remaining crashes can be traced to either bad hardware (like a failing power supply or a bad RAM chip) or buggy software that runs in kernel mode (like an antivirus program or a CD/DVD-burning application).
We spent a lot of time in Windows XP Inside Out explaining how to track down the cause of a crash. Most of the time, it can be done, and fairly quickly.
(Thanks to Scoble for the pointer.)