How to Fix a Slow PC.
1. Defrag Your Hard Drive
  Your computer stores data out of place physically on the disks to speed up the process of writing data.  Over time this can get out of hand and cause the hard drive to work more to recall a certain file.

2.Rid your computer of any malware that may be on it. 
  Malware can be as bad as a full trojan delivered to you by a hacker or as simple as adware witch is just a nuisance and slows down your computer either way it has no business being on your computer.

3.Free hard drive space
  Verify that there is at least 200-500MB of free hard drive space. This available space allows the computer to have room for the swap file to increase in size, as well as room for temporary files.

4.Update Windows
  I'm not saying you need the latest and greatest but Make sure you have all the latest Windows updates installed for your computer.
If you are on the Internet when your computer is slow, make sure all browser plugins are up-to-date. You can also try disabling browser plug-ins to see if one of them is causing the slowness.

5.Memory upgrade
  This is usually only true for computers with a little age on them.  Over time modern programs are utilizing more and more memory if you don't have at least 2 Gigabytes of memory it may be time to upgrade.

6.Background programs
  Due to programs demanding more memory; an unnecessary amount of programs/processes running at once could be an issue especially if you don't have that much memory.