With Windows you tend to add more and more programs and services during the lifetime of the computer, as you try to get more and more pieces of hardware and assorted technologies up and running on the system. Each new layer of software encrustation tends to stay - because after a while you can no longer tell if you actually need a given program. Virus scanners, firewalls, printer monitors, software that looks after your sister's scanner(s), it all adds up - and after a while you notice that the machine is not as fast as it used to be. Well, to add a little science to the exploration of what is causing the most significant problems here is a great link: http://www.thepcspy.com/read/what_really_slows_windows_down. If you follow the link you will find a detailed analysis of different virus scanning programs and their effects on boot time, computational performance and disk access times. The results are very interesting and the methodology can be applied to any programs or processes that you might find questionable on your own machine. Of particular interest is a simple timing program which checks the arithmetic performance of your CPU and times disk access on your machine. This is a recommended article for anyone with an interest in keeping their machines running at a reasonable rate.