Bitdefender CryptoWall Immunizer is a simple portable tool which can help to protect you from versions one and two of the Cryptowall file-encrypting virus. (It may work with a few other threats, too.)
The program couldn't be much easier to use. Launch it, click the "Immunization" switch to turn it on, and wait until the green "Protection Enabled" message is displayed. That's it.
CryptoWall Immunizer doesn't have to run in the background to be effective, so you can now close it and get on with the rest of your day.
But wait. The program isn't doing much - just changing system policies to prevent executables from running in the %appdata% and %startup% folders - but that could still be enough to cause problems with other software. Maybe serious problems, depending on what it affects. Make sure you know how to reboot into Safe Mode, or take other troubleshooting steps if your system won't start properly, then reboot your PC and test your main applications, confirm everything works as it did.
CryptoWall Immunizer is a very basic tool which offers only very limited protection. It's still worth using, but is no substitute for a real antivirus program.