The things that slow a computer down are insufficient memory (4gb is OK for 32bit systems but not for 64 bit systems), hard drive fragmentation, hard drive is mostly full and applications that do not release resources.
Add as much RAM as your computer can use. Most computers have a max limit and any memory above max is not usable. Most of the time the number of slots available also limit memory, check with the manufacturer of the computer/system board for information on what memory is supported.
Hard drive fragmentation happens all the time, a document is edited and saved tends to get fragmented over time and the slow down happens when the hard drive has to hunt to connect all the fragments for an application. Run Defrag tool from system properties of the hard drive. Also check the amount of free space on the hard drive, when the drive gets over 90% full it will be harder to defragment the hard drive. Delete the trash, use the drive tool to help find things to remove. Archive files to an external drive or cloud service, remove unused applications.
One more thing that makes computer run slow is usually the browser. Each tab for a browser takes memory resources and having too many tabs will slow a computer a lot. Also opened applications that use a lot of resources should be closed until needed.