I will use the Acer Aspire laptop computer for example. Each laptop hard drive has a hidden space for restoring your computer. If your computer will not get past the user sign on there are a couple of things to try to restore your computer. Unfortunately, you will lose all of your information that you had on their prior to the recovery process. Pictures, documents, etc.
This is a good reason to make sure that you make back up discs for your computer.

Step 2
Use your User's Guide for your particular model if you still have it. Another good thing to keep around.
Find the section in your guide book on system recovery/restore. For the Acer Aspire, to begin the recovery process, simply restart the system. If you are at the point where you cannot log on and click the restart button, just turn the computer off. Once the computer is completely off, turn it back on. While the Acer logo is on the screen, press the Alt key and the F10 key simultaneously. This will begin the recovery process.

Step 3
Once the screen goes black you will be prompted to complete the recovery with onscreen instructions. Once this process is done, your computer will restart and Windows will start over as if you had just bought the laptop.