An epidemic of uninvited guests
One of the recurring problems I encounter is spyware and adware. When helping home or other casual Windows users with performance problems on their Windows XP systems, I frequently encounter programs that have installed themselves without the users' knowledge - invariably, I hear "I have no idea how that got there."
Upon inspection, I often find a variety of 'hidden' program set to run when the computer is started. These unwelcome and uninvited
guests pests bring a variety of problems with them, so it's always best to eliminate them from the computer whenever possible.