Through excessive use a PC can become quite crowded with useless information, which takes up space on your HDD and consequently slows down the speed at which your machine operates. Much of this information comes in the form of what is called "cookies" and is obtained via accessing the web. These cookies are generally harmless other than the slowing effect they have and can easily be wiped from your HDD. Sometimes access to the web may result in your computer acquiring a virus. A virus is a piece of software o programs that seeps into your system and attaches itself to files on your HDD. Every time that these virus-ridden files are executed, the virus has a chance to reproduce or duplicate. The magnitude of the damage a virus can cause through a single period of magnification is enormous and can have detrimental effects on your PC. In some instances the virus can go so far as to render your system completely useless. Viruses can travel from user to user incredibly fast as well. Some have been know to infect millions of PC in a number of days. If your system somehow contracts a virus, the process of removing it may be quite rigorous and likely result in the loss of the information on your HDD. You may be lucky enough for the virus to not have progressed far into the inner workings of your system though and in such a case it may be removed without extreme difficulty. To lessen your computer's chances to obtaining a virus you should invest in protective programs as this can help you defend against infection without as much concern.