U know that ur computer is infected when :
Your computer freezes frequently or encounters errors.
Unusual system messages or fake error messages appear.
Your computer slows down when programs are started.
Microsoft Internet Explorer freezes or functions erratically.
Your computer automatically shuts down after displaying a System Shutdown message.
Registry entries have been added or removed to execute unknown programs at startup.
You receive notification from your firewall that some applications have attempted to connect to the Internet, although you did not initiate this.
The most common causes of these symptoms are :-
1) Spyware/Adware - is computer software that collects personal information about users without their informed consent. To know more about how spyware attaches itself to computers and to learn how to combat them, CLICK HERE
2) Virus - is a program written to alter the way a computer operates, without the permission or knowledge of the user, it hides in other program files. It replicates itself and executes.To know more about how viruses and to learn how to combat them,CLICK HERE