Cryptowall and its variants are nasty little viruses that not only hold entire computer hostage but encrypt all your documents/files as part of the process. Only a proper backup can restore the files once the infection is in place. I can get rid of the virus but I can't undo the damage it does.
Prevention is the best medicine in this case. While there are others out there that do the same thing, Bitdefender's got a good reputation as an antivirus company and this program is dead easy to use. Just download, install and, once running, click the "Immunization" button. This blocks the running of programs form a couple of rarely-used folders where Cryptowall always runs. In the rare instance that you've got a program that runs from one (e.g. Chrome is an exception but this software knows about that), you can always run the program again, deactivate the immunization, and turn it back on later.