Lots of things can slow a computer down
- Physically dirty computer
- Failing hard drive
- Malware
- Too many things running on the computer
For the first item you can simply look inside the computer. If there is an excessive about of dust and dirty this should be cleaned out either with canned air, air compressor or a special anti-static vacuum.
Second item can be determined by downloading and using a DFT (drive fitness test) application such as Seagate SeaTools
The third item can be hard to take care of if you don't have the knowledge or expertise, but for starts do you have any security software running on your computer? If you don't you definitely should have something. There are many free ones such as Avast that...