There is a currently a particularly nasty form of malware circulating the internet dubbed “ransomware”.
It can completely wipe out a computer, plus any external drives or network drives attached. Once infected you are effectively locked out of your PC and files. All your computer does is show you a message about paying a large ransom in a crypto-currency like BitCoin. The ransoms I have seen have been for US$500, but there is no guarantee that paying the ransom will undo the damage.
Infections are usually caused by opening an email attachment, but the malware can also be disguised as music, movies or software from torrent and download sites.
Prevention is important as recovery is expensive and some or all of your data could be lost permanently. Once infected, the only guaranteed cure is to recover from a backup. If the backup is connected at the time of infection it is likely to be lost too.
Unfortunately, antivirus software cannot stop all threats. Some estimate that more than half of malware attacks are able to bypass the best antivirus software. Those who produce the malware are always 1 or 2 steps ahead, and there is so much money involved now that they are very motivated and organised.
The best prevention is to never open any attachment that you are not expecting, and to be very careful with which websites you visit. Be particularly suspicious of emails purportedly from Australia Post and other delivery companies, and bank or government agency messages. If in doubt, call me or check with the sender via an independent communication method (ie don’t use the email address or phone number supplied in the email).