Why is my computer running slow?

Once you've had your computer for some time, say, a a couple of years, you may notice that your computer seems to work slower than when it was new.

On of the reasons this happens is file fragmentation, which is a normal result of storing programs and data on the hard drive. If you are a heavy user of the computer, de-fragmenting (or defragging) your hard drive should be done once a week or twice a month; depending on how much data you are storing on the hard drive, including installation or deletion of programs.
If you are a light user of the computer, then you need only defrag the hard drive once every month or two.

Another reason your computer may slow down (that doesnt involve viruses or spyware) is the amount of programs you have installed. If you have taken up more than 50% of your hard drive with programs, your computer will run slower simply because the read/write head on the hard drive has to travel more, which takes more time.