Microsoft describe spyware as "... a general term used for software that performs certain behaviours such as advertising, collecting personal information, or changing the configuration of your computer, generally without appropritately obtaining your consent."

Signs that you might have spyware include:

• You see pop-up advertisements even when you are not on the Internet
• Your browser's home page or search settings change without your knowledge
• You notice a new toolbar in your browser that you didn't want, and find it difficult to get rid of
• Your computer takes longer than usual to complete certain tasks
• You experience a sudden rise in computer crashes

Microsoft provide more information about what spyware is in this article