Other than being so familiar with your personal computer that you will be able to notice when it is behaving in a strange manner or that it seems to have become sluggish, the only way to tell if you have a virus is through the use of a virus detection program.
Virus detection programs use an existing database of virus patterns called a “virus definition file” to search your personal computer for viruses. By comparing the information on your computer’s disk to the virus definition examples, the virus detection program is able to detect portions of data that match certain virus patterns.
Once the virus pattern has been determined, the virus detection program (also referred to as anti-virus software) will let you know that a potential virus has been discovered. You will then be given the opportunity to delete the virus. In most instances, this simply involves clicking a button on the screen that will tell the anti-virus software to delete the offending file.