?If our computer behaves unexpectedly often, then one of the reasons may be our computer is infected with some virus, adware or spyware.Computer virus is a man made program that enters in to our computer without our permission and does the unwanted things. Some viruses may enter in to our computer and duplicate themselves by occupying all the empty space in the memory, and some may manipulate even the security systems.Some of the malicious programs other than virus programs are worm, Trojan etc. which may also cause significant damage to the computer. In order to protect our computers from the damage caused by these malicious programs we can install anti-virus programs in our computer.These anti-virus programs scan our hard disk for viruses, Trojan etc. and delete them. Example of Anti-virus program: McAfees Stinger tool, which can detect and delete many of common viruses, Norton Antivirus etc.Spyware is an executable program that secretly collects the information about the internet users and sends this information to somebody we dont know.They may misuse our personal information for advertising purpose or sell to some third parties. It secretly sends the information like which sites we are visiting, credit card numbers, e-mail addresses in our address books, passwords etc.Spyware can alter the home page in our internet browser, scan our hard disk files, watch keystrokes, read cookies, enter in system registry etc.Adware is a software package that automatically displays advertisements when it is installed in our computer. Sometimes it comes along with the freeware software packages we download from the internet.Because of spyware/adware, some of the advertisements pop up unexpectedly and our internet browsers home page may be altered by an unknown site. As they use all our system resources for sending this information, we may experience some lagging or instability in our applications when it is installed in our computer. Computer virus, spyware, adware is a man made program that enters in to our computer without our permission and does the unwanted things. In order to protect our computers and computer maintenance from the damage, we can install anti-virus programs in our computer.