DNSChanger is a computer virus that, in layman's speak, affects your PC's ability to accurately navigate its way around the internet. In technical terms, the malware has the power to change your PC's Domain Name System (DNS) settings, which gives the virus's creators the ability to redirect you to any number of fraudulent web sites and other places you basically don't want to go.
Luckily for us, in late 2011 international law enforcement authorities succeeded in shutting down those responsible for the malware and apprehended the servers they were using to reroute victim's PCs, but this in itself didn't eradicate the malware on users' PCs. To this day, potentially hundreds of thousands of internet browsers are still being rerouted via these servers -- only now they're being legitimately redirected to the sites their users originally intended to visit.
All of which brings us to next Monday, July 9. On that date, the servers in question are being shut down, which means if your PC is infected with DNSChanger (and you're still being redirected to and by those servers) you won't be able to surf the web. Fortunately, there's a very simple way to determine whether your PC is infected with DNSChanger. Back in March the Australian Government set up dns-ok.gov.au, a web site which diagnoses whether your computer is affected by the malware. It only takes a second to check, and the site provides further information on what to do if your system has been compromised.