I can tell you from experience that there are few things that are more annoying. When this first happened to me, I wanted to throw my computer out the window! But fortunately, I did a Google search instead and found out about a much better option
Here are some of the options you have if you want to get your slow computer fixed
1. Do a reformatting of your hard drive and reinstall Windows
This option requires locating Windows backup discs that you got when you bought your computer. If you cannot find those discs, you might have to get them from Microsoft. Make sure that you backup any files you need, since they will no longer be there once you have re-formated your hard drive
2. Get your computer repaired at a shop
This is a very expensive option. It will probably set you back at least a hundred dollars, maybe more, depending on how much labor is performed. And you may not even get your computer back until two weeks later
3. Repair your computer at home
This is my favorite option, and the best overall. You use a program that goes in and finds all of the errors that are slowing down your computer. Those errors are then isolated and removed, and the program will also make sure that they do not return in the future. This process also does not take very long. Within 30 minutes to an hour, your computer can be running the way that it did when you first got it.
Source: ezinearticles.com