One of the things that happens to many Windows users is that their computers end up slowing down over time. Much of this is attributed to virus issues, malware problems, and registry errors. There are programs to deal with each one of those. One problem that a user causes on their own is not knowing what programs are running on their computer at all times.
To find out what programs are currently open and running, the easiest way is to hit Control + Alt + Delete. Push these and hold all three down at the same time until you see a dialogue box come up. In that box click on "Processes". This brings up a list of the applications and programs that you are currently running.
Before you do anything be careful that you don't accidentally shut something off that is actually needed to keep Windows running properly. If you do shut something off, you will have to reboot to fix the problem.
Sort the programs by their ram usage to see what the biggest computer speed hogs are on your machine. You might notice quite a few applications are in your active RAM that you aren't currently using. Some of the usual ones are Microsoft Office running in the background or the iTunes Updater. You can end these processes to free up some RAM since you don't need these things running all the time.
Another important reason to look to see what is running is because something viruses and malware will show up here. You can see them open and running while using your computer. Many have become smarter about not showing up in this list but some still do. At times these can also be caught by looking at your bandwidth to see if these programs are sending or receiving data.
Once you understand what is going on with your machine, you will be able to maintain it much easier and help it run more efficiently.