How spyware is installed in a computer without giving alert to users?

How spyware is installed in a computer without giving alert to users. As per my knowledge from programs like asp, aspdotnet or java. we cant access the client side system. But some site says with javascript a spyware can be installed. How and Why ?

Answer:

Spyware is any technology that aids in gathering information about a person or organization without their knowledge. On the Internet (where it is sometimes called a spybot or tracking software), spyware is programming that is put in someone's computer to secretly gather information about the user and relay it to advertisers or other interested parties. Spyware can get in a computer as a software virus or as the result of installing a new program....

And

Spyware will be installed in terms of Cookies Files and wen u open a site a pop up window will get open Like Ad's..Some of this type of Ad's Will be saved in your PC without ur prior notice..So, Just use Some anti spyware tools n also Block pop ups in browser..

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