A virus is a piece of code that gets loaded onto your computer without your knowledge and runs against your wishes. The first known occurrence of viruses goes back to 1987 when the ARPANET was infected by one.
One common misnomer among people is that you can infect your computer just by opening an email and reading its text. That is not possible; it is usually the files attached to the email that contain the virus. The most common file types are ".SCR" ".VBS" ", ".PIF"
Prevention is better than a cure: Here are some tips to make sure that your computer does not get infected.
Get a good Anti-Virus software like Norton Anti-Virus, MacAfee, PC Cillin etc.
Keep your anti virus software updated by downloading new virus definitions regularly. Most Anti-Virus software comes with the feature of updating virus definitions.
Keep your windows operating system updated by regularly downloading new updates from the Internet. Windows O/S's have a lot of security loopholes and bugs that can be easily exploited by worms and viruses.
Make sure the anti virus scans the file each time before its opened.
Floppy disks and removable media are a good source of viruses; always scan them before accessing files on them.
Never open email attachments from sources that are unknown or suspicious.
Do not open emails that have questionable subject lines.
When in doubt about a file, don't open it.
Even with the best of precautions bad things can still happen. Backup all your data and important files.Even after taking all these precautions if your computer does get infected, then here are a few things that you can do:
Online clearing tools - are a good source of trying to clean out the virus, there are many Anti-Virus websites that offer free online virus detection and removal services.
Removal tools - If you have an Anti-Virus software then you can go to the website of that software and download removal tools designed specifically for the virus. However, you must find out the name of the virus that infects your PC beforehand.
If you have a backup of all your important files, you can also consider formatting your hard disk.