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.
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.