Viruses, worms and trojans often exploit errors or bugs in your computer's software. These exploits, usually unintentional and complex, are hidden ways in to the core of the operating system or application, and allow the malware program to access parts of your computer that are normally blocked.

The manufacturers of operating systems, such as Microsoft and Apple, work hard to close these gaps in their products' security, and often release updates or patches that fix them. However, to take advantage of these fixes, you usually need to download them from the web - this can be done automatically in Microsoft's case at http://windowsupdate.microsoft.com/