Your hard drive might be full. If the hard drive stores much data than it possibly can( Usually if there is under 200mb free space left, it could be considered dangerous ) . It will slow the computer.
To prevent this from happening, please remove the excessive software that is currently installed on your hard drive, when space is opened the computer will turn back to it's normal state again.
In Windows :
To do this simply delete files , Cleanup windows from : My Computer --> Right Click at your hard drive --> Choose properties and click Disk Cleanup located in the properties window. And choose your files to delete.
Also, Go to Control Panel. And click at " Add / Remove Files. This is a list of files you've installed on your computer and you may delete the ones that you would like to delete.
Secondly, Your computer might be infected with a Virus or a Spyware. This will slow the computer excessively. To get rid of the spywares or viruses you will have to use an anti virus or a spyware program.
You can alternatively Format your hard drive and reinstall a fresh copy of an operating system or Change your hard drive by buying a new hardrive.