Nothing exists that will stop it because it doesn't activate until you click on something, anything on the page it generates.
Best to immediately power off then back on into Safe Mode and initiate System Restore to before it all happened.
There are a few tools to help in the last link in my signature below that may help. Try Stinger and Malwarebytes Free.
Well, it's not a virus but a Trojan, which makes prevention of this ransomware a bit more difficult. A lot depends on how it arrives. It could be the result of something downloaded that isn't what it pretends to be, or a web page that's had extra malware code inserted could redirect you to a site hosting a BlackHole Exploit Kit - which will scan a PC to see whether it has old versions of Java, Flash, Reader or a number of other programs. If it has, there are many program flaws (patched in the latest versions) which can be exploited to download this and related malware.
The best protector is to keep all your third-party programs fully up to date, along with McAfee, Windows and whichever browser you're using. There are things like Microsoft's EMET which will help keep you safer.
Ultimately the only way to be (almost completely) safe is to have a whitelist of allowed programs and to prevent anything not on the list from running. The Enterprise versions of McAfee allow you to do something like that.
Most antivirus companies try to keep out malware by looking for signatures of known malware. The authors of these programs of course keep changing the programs in various ways so as to change the signatures.