If you use the internet alot, and download songs from Kazaa and LimeWire, Spyware can build up on your computer. A few spware is harmless. But alot of spyware can be dangerous, as it can get personal details, and worst of all, it is very annoying. Your internet and CPU will run slower, and you will get constand pop-ups, whether it is Search Assistantants or browser modifiers. Spy-Bot Search and Destroy is good for Spyware, and Ad-Aware (Lavasoft) is good for Ads. The best one I'd say can be downloaded on the Microsoft site. It is Microsoft Anti-Spywar Beta. It does Adware and Spyware. But you can only download it if you have Windows XP