If your computer has been infected with a virus you will quickly see odd signs that it exists. Strange things start happening that you cannot explain. Some files might move around, odd error messages may/will appear, programs might start to fail. For example if your virus scanner program stops working then it could have been disabled by a virus. With all this action taking place in the background your computer will also start to run slow and seem like it is busy all the time.
When browsing the internet you might find that you will not be able to visits the pages that you choose. You will be redirected to other websites. Your computer might also be sending emails to everyone in your contact list without you knowing it.
Most people think that a virus simply attacks the file systems of your computer, however this is not true. Some viruses are designed to damaged computer hardware components until they are ruined completely such as hard drive and video cards.
Many people will argue that a virus only infects software, however all hardware is controlled by various types of software. In fact the entire computer is controlled by a BIOS which is software. The BIOS controls the temperature of the motherboard in which a viruses will/may change temperature control on your motherboard. Therefore, the motherboard would overheat and fry itself.
If you suspect that your computer has a virus, scan your computer asap with an antivirus program. If you do not have one or yours is not working, download another one, or get a free scan using the free online virus scanners.
If you find viruses but cannot remove them you will need to take your computer to a professional before you lose everything. Another option is to format your computer which means you loose all the files on it and re-install a fresh copy of the operating system. You can backup your files first, but please be aware that they might be infected with a virus also.