Start with cleaning off cookies and history from your browser.
Also if your still using internet explorer, consider switching to chrome or firefox
Use the disk clean up tool, and maybe a defragment tool if it needs it.
Consider what kind of anti virus you use. Some anti viruses use more resourses than it should, on older computers that slows it down tremendously. McAfee is the worst culprit in that catagory. Norton or Avast would be what i recommend. Avast is completely free for a year.
Clean up by organizing the random files just sitting around into folders.
Go through your programs in the Add/Remove list and look for programs that you don't use. Anything from Microsoft don't uninstall, usually those are important, but you should be able to tell which ones you installed.