Understanding the troubleshooting process is vital to getting a system up and running as soon as possible. Without going through a specific process, it becomes difficult to determine where the actual problem may lie.
Write down everything you know about the machine. Keep a record of what goes on and what is tried at least until the problem is solved. Ideally, keep the record after that in case it ever happens again.
Below are a list of questions that should be answered before any other steps are taken in troubleshooting a Windows machine:

1. What is wrong? "The computer doesn't work" isn't good enough. "I'm getting a General Protection fault every time I start Word Perfect" is better. Without knowing the exact problem, it's like finding a needle in a haystack. Get a detailed description of the problem.
2. When did it start happening? Was it first thing this morning, last night, or has it been going on for a number of months? Is this a new installation of Windows?
3. What has changed? This is very important! 9 times out of 10, what's changed is the root of the problem, and changing it back will fix it. If it can't be changed back, there is at least a place to start.

Once these questions are answered, you'll already have the beginning of the troubleshooting process done. From that point, the error will need to be isolated and a fix found. The rest of this Web area is dedicated to explaining different troubleshooting areas and how to use them.

Good luck and enjoy!