There are many ways of knowing that your PC has caught an infection from the Internet or elsewhere.
An infected computer will behave differently from its normal state. You may not be able to boot your system, or it may slow down to a crawl. You may see strange or unfamiliar messages or graphics on your screen, and you may be unable to reverse these changed display settings. The programs that you use everyday may start acting funnily.
There may be a sudden rise in the amount of data traffic going out of your PC, even when you are not actually on the Internet. Files may start disappearing from your hard disk, and your machine may start sending out spam or other malicious emails to all your email contacts without your knowledge and consent.
Viruses can be hard to detect initially. They rely on stealth to gain access to a host PC, and are more likely to work quietly in the background rather than give you any immediate indication of their presence. Things like your system slowing down or your hard drive space being chewed up can happen for a number of other reasons as well, so most people do not immediately think of them as being caused by a virus.
The minute you begin to suspect that your PC is infected, you should take it off the network—local as well as the Internet. It is best to unplug the network cable altogether. This will keep your machine from attempting to spread the infection to other machines that may be connected to it.
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