A computer virus is a small software program which can spread from one system to another system. It can corrupt or delete data on the system and also it can use an email program to spread the virus to other email addresses in the online address book. These viruses can even delete every information in the hard disk. More than 5000 new viruses are released every month. These viruses can cause harm to the computer by deleting the files, reformatting the hard drive or damages the programs. It slows down the performance of the system and also can cause frequent crashes.