They can steal your internet passwords, files on your computer, credit card number etc.

The biggest difference is that a trojan is a program that covertly (secretly) enters your computer, send something on or downloads something without your knowledge.

Spyware are programs that basically spies on you. It finds your passwords, sites you have visited etc.

You are right in the fact that you don't want them.

These things come from websites that have a hidden line of code that makes the virus/trojan/spyware (hosted somewhere else most often) download to your computer and begin to do what they do (described above).

One of the best anti-spyware programs (programs that removes spyware or block the download) for beginners is Windows Defender. The program is made by Microsoft and can be found here:

http://www.microsoft.com

Check below the picture to the left and you will find "Windows Defender" Beta 2. Click on it, and further installation information will be displayed.

Let me know if there is anything else you want to know :)

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"The larger the island of knowledge, the longer the
shoreline of wonder" (Ralph W. Sockman)

For spyware, malware and adware you can get either Ad-aware SE Personal or Spybot Search and Destroy.

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Welcome to My World
They say blood is thicker than water
I say, cum is thicker than blood and water. Do the math.
There's an ice pump where my heart is supposed to be.