Computers can start running slow for a variety of reasons. Your hard drive could be getting full, the drive could be fragmented, over heating, or faulty ram could all cause problems with slowness. The first thing I would do would be to clean up your hard drive. You will want to clear out temporary files and cookies from your computer.
I use a program called CCleaner to do this. It will clear out old temp files, temp internet files, and cookies to free up space on your hard drive. Another thing that can cause slowness on your computer is having too many programs running in the background. CCleaner has a feature which will let you disable programs from running at startup. You could also do this by going to the run command and running msconfig to do it manually. If this doesnt help, you should look at doing a defrag of the hard drive. As drives get used, files on them are written with their parts further apart, which makes the drive have to search more when they are called on, which then turns into slowness as you wait for the file to come up. If you right click your My Computer icon, choose manage and then there should be an option for disk management. In there should be an option for disk defragmentation. This should reorganize the files on the hard drive so the parts are written closer together so there is a less of a wait time when it searches.
Another thing you should do is run antivirus scan and also a malware scan as viruses and malware can slow down performance. Use antivirus like Norton or Avast!(which is free) and run a malware scan with something like SpyBot Search and Destory or Malwarebytes (both are free) to clean out any bad things that may have gotten on your pc.