Formatting of Hard Disk and Installation of Antivirus Software

What Antivirus is best suitable for my Desktop Computer and what simple steps can I follow to format my Hard Disk? After starting my computer (when the desktop shows up), it runs very slow and freezes at most times. I suspect these happenings caused by the activities of virus. I did defragment the hard disk of my desktop computer but to no avail. However, I also don’t think the sluggishness of the machine has the anything to do with the speed of the CPU or RAM size.
