A virus is typically a small piece of code that comprises a software program that works to cause your computer to malfunction. 

Some viruses are simply designed to be a prank, while others have been engineered to be system killers. 

More recently, malicious programmers have started to use viruses as a tool for mass identify theft scams that can result in the crooks gaining access to your bank accounts, and dozens of other personal data entries found within your computer files. 

There are numerous ways to become infected, with the most common causes being email attachments, file sharing and adult websites. 

99% of all viruses are designed to attack Window's based computers - which isn't to say that other systems such as Macs aren't vulnerable, it is just that with the overwhelming majority of the computers in the world running Windows software, it's not worth the time to design viruses that attack less popular operating systems.  

Over the past few years, Microsoft has taken numerous steps to increase security in XP as well as their new Vista operating systems. 

In response, hackers and malware programs have taken to exploiting weaknesses in universally installed programs such as Winzip, Itunes and Adobe PDF readers. 

Most people don't realise they have a virus until their computer starts to run slowly, even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt.

While it is very easy to have an Anti-virus protection from the threats, it is a much more complicated task to 'clean' a computer that has been affected, as the saying says "prevention is better than cure".