Your question is simply too general to give a simple answer. The computer, Windows or a program might crash for various reasons, to tell why is a bit more complicated than simply tell you "Do this to find the reason".
If it is an application issue as for instance a game crashes or do not run properly, you need to check that you have installed it properly and that your hardware meets the minimum requirements for that particular application. Quite a many applications have their own support forums where you can then find help, especially if the crash or issue gave you a clear and valid error code or message.
If Windows itself crashes you find excellent instructions on how to ask help here at the Seven Forums, made by our admin z3r010:
http://www.sevenforums.com/bsod-help-support/96879-blue-screen-death-bsod-posting-instructions.html
In all other Windows issues, search or browse our forums to find if the issue already has existing threads, if not you can start your own thread explaining the issue. When doing this, be as accurate as possible telling at least:
- What happened
- What happens when the issue occurs
- What did you do / were doing when the issue occurred
- Any error messages you got
- What have you done to fix the issue
Also fill in your system specs as completely as possible; especially in hardware related issues it is important to know your hardware setup in order to be able to assist you.
Usually we are quite good in finding the underlying reasons to any Windows issues. I have been around here and at our sister forums the Eight Forums over 5 years now and only my
last issue has not been resolved yet, meaning I have always found help and solution here.
Kari