Viruses can do some real damage to your computer.
Some viruses can go in to your system and completely lock down your computer. With some instances of hidden items, redirects, file association corruptions, and pop up ads that lock you out.
A lot of viruses hide when they get in to your computer. One of the main things they do is hide behind the anti-virus software that is installed on the computer. This is why when you run scans to try to get the virus off of your computer it never shows up as being in the computer. With the tools that we use we can get that virus to show up on our scans so it can be dealt with properly.