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.
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.