It’s very rare for Windows 7 to fail. Windows will fail on its own only if something disastrous happens, such as a power surge or a sudden reset while Windows is modifying a critical system file. Problems are more commonly caused by something outside of Windows, such as software, updates, and drivers. Physical hardware almost never causes problems in Windows.
A great many of the problems with Windows are caused by poorly written software or hardware drivers or by having too many devices or programs installed on or in your PC. In my experience, the most common problems within Windows are caused, in order, by:
1. Device drivers
2. Poorly written software
3. Poor security
4. BIOS corruption
The Domino Effect
Some problems can cause what is called a domino effect, where one event sets off a string of other events, so it’s always advisable to diagnose and repair problems as early as you can after they first appear. One unchecked problem can then lead to others, because a malfunctioning process, service, or driver can cause other programs or Windows functions to fail, since these processes, services, or drivers are often shared by several applications or Windows components. For instance, you might have a problem with Internet Explorer crashing repeatedly. This could be because a component Internet Explorer shares with another Windows program, such as Windows Explorer, is corrupt or because another program or process is causing it to crash. The point is that the source of a problem is not always obvious; a failing program may not be the root cause of the issue. In these cases, you can use more advanced diagnostic methods and tools to diagnose an issue.
Source of Information : Microsoft Press - Troubleshooting Windows 7 Inside Out