1) Get us the exact phrase of the error message. If it is the blue screen, you don't have to write it all down but make sure any of the files mentioned are exact. Information like "it said something like can't install" isn't very helpful. Information like "it said installation failure code 2A9905 can lead us directly to the problem.
2) What were you doing? If we know specifics about what you were doing we can sometimes find the program that is causing the difficulty.
3) Be as accurate as possible in describing the computer's state. If you tell us it is doing nothing we may assume that the computer's lights don't even come on. If you say that it gets to the log in screen but won't get into Windows that is much more helpful.
The more specifics you can give us, the faster we can help. Many times, good information can lead to us giving you an answer that will take care of it for free!