Maybe , Recycle Bin is full. or the history of navegation is very charged. Try to don't have many sofware start and install only the software that you knows.
Due to virus in the computer it may cause of working slow. Scan your computer through any antivirus if any virus is found then try to delete all. if there is no any such kind of problem or virus in your computer then it's better to install your windows again.
See how many icons are in the bottom-right of the menubar near the clock. It could mean a lot of programs are running. Try right-clicking on the icons and choosing "exit".
I understand your problem for that i would like help you regarding to your problem.
If your computer become slow than very First you remove temporary files on C:\windows\temp location. And also keep some free space in your each drive also Use hard disk utilities software. That is available free on internet.