First before you start, get your computer into safe mode. To do so, restart your computer, then press F8 continuously. Select the safe mode option, then continue to desktop.
One way to make your computer faster would be to go through all of your files and folders and delete everything you don't need or want such as programs, pictures, videos, or anything that may be harmful to your computer or that you are not sure about. After you have gone through your computer, go to start, click on control panel, and click "uninstall a program". This will take you to a list of all of the programs that are installed to your computer. You can uninstall all of the programs you don't use or could be making your computer slow. Once you have done that, go to your recycle bin and empty it.
Next, you should install a Spyware program. These are free, and they will get rid of all the bad files or viruses in your computer. They will also prevent any future spyware from entering your computer. You can go here to download Windows Defender: http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
After, locate the temp folder in your computer and delete everything in the folder. This is located, usually, in C:\Documents and Settings\Username\local settings.
If you follow these steps I can almost guarantee your computer will go faster.