Here are the most common causes of poor performance.
Music Sharing Software - poorly configured sharing software can use up a lot of network capacity and fill up your hard drive. It also often comes with spyware.
Web enhancements - animated add-ons like Bonzi Buddy and Comet Cursor can slow down even the best computer system. These programs also come with spyware.
Spyware - extra software that comes with freebies like KaZaA, Bonzi Buddy, Comet Cursor, and many other downloads. It installs itself invisibly and runs without your knowing it, tracking your web surfing habits, popping up ad windows on your monitor, and just generally ruining your system's performance. Run Ad-Aware regularaly to keep your machine free of spyware.