So if your computer is having boot or startup problems, the first thing you'll usually want to do is boot to Safe Mode. If you can get to Safe Mode, then often times you can use System Restore to go back to an earlier time when the computer was working fine.
So how do you get the computer in Safe Mode? First, turn the computer off (use the power button the tower if you have to). When you turn the computer back on, you'll want to keep hitting the F8 key on the keyboard until a menu comes up. In the list of options, you'll see Safe Mode. Use the arrow keys on the keyboard to select it and then hit the
Booting to Safe Mode takes a few minutes and you may see a black screen with a lot of text on it for a couple of minutes. Just be patient and wait. After a couple of minutes you should see your computer desktop again (only it will look huge and colors will be off - that's normal).
If the computer never progresses past the black screen with writing (be sure to give it a good 15 minutes) - then you've got bigger problems to worry about. You could try repeating the process to boot to Safe Mode and see if it works next time or you could try "last known good configuration" in the startup menu of options when you press F8.
If neither of those work, then chances are you'll have to reload your computer from scratch again (but make sure your data is backed up first).
Knowing how to boot to Safe Mode can help you solve some computer problems so it's good to know how to this.