A Virus is a code written with the express intention of replicating itself. A virus attempts to spread from computer to computer by attaching itself to a host program. It may damage hardware, software, or data .
Adware is any software package which automatically plays, displays, or downloads advertisements to a computer after the software is installed on it or while the application is being used. Some types of adware are also spyware and can be classified as privacy-invasive software.
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While most spyware and other unwanted software come bundled with other programs or originate from unscrupulous Web sites, a small amount of spyware can actually be placed on your computer remotely by hackers.
Installing a firewall or using the firewall that's built into Windows XP provides a helpful defense against these hackers .