os is too slow

Solution
If it's installed correctly and your complaint is about startup time or time to launch programs the best solution is to replace the hard drive with an SSD.

If you have a laptop and only space for a single drive, consider the SSHD from Seagate which is a combined SSD and laptop. It auto-configures so the boot and certain other files stay on the faster SSD component.
If it's installed correctly and your complaint is about startup time or time to launch programs the best solution is to replace the hard drive with an SSD.

If you have a laptop and only space for a single drive, consider the SSHD from Seagate which is a combined SSD and laptop. It auto-configures so the boot and certain other files stay on the faster SSD component.
 
Solution
You need to add some information to your post. What model/brand of computer is it? What are the system specs, CPU/RAM/Video card especially? Was it always slow? Exactly what is slow? Programs, internet, startup, movies playing, what? Viruses, installing a lot of junk software off the internet, spyware, slow computer, bad hard drive, lots of things can cause a slow system.