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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How can I keep my pc virus-free without slowing it?

This is a question posted on Yahoo Answer by Farnaz Ahmad

I used many anti-virus soft,but none of these worked properly.I am facing some problems with explorer and file extension.In addition,those soft made pc slower.Is there any better solution without slowing the pc too much?

Steven25:
suggestion:

do disk clean up and defragmentation

disk clean up computer>hard drive c:> righ click properties>general tab> disk cleanup> tools tab> disk defrgmentation.

and use Ccleaner.

last solution: back up all data and do clean installation of your OS.

ED G:
Make sure your computer is tuned up. Most anti-virus run pretty light now days, and use way less ram than other programs. For instance I'm using Norton Internet Security and it's using 10mb of ram. While firefox is using 140 mb or ram. The only time a anti-virus should slow down your computer is when it's scanning for virus. Which you should do when not using the computer.
Now it is true that some of the anti-virus programs will slow browsing some, because of real time web protection. But not a lot, if a page takes one second to load, with some anti-virus it may take a second and a quarter. One quarter of a second isn't bad really. Gone are the days when anti-virus programs used a lot of ram and slowed down a computer. Some still do, but overall most run light, Norton, Avira, Microsoft, Nod32.