I've been conversing with Peter via PM and I'm quite confident it's not CryptoLocker (if it was this would be a doddle!). However, I think I've established it's the TeslaCrypt ransonware that has been identified recently. TeslaCrypt is a new variant on the CryptoLocker theme but it encrypts your files with the extention *.exx which is the case with Peter's files. Thankfully, it's not the latest strain, AlphaCrypt because, with TeslaCrypt, there is a way to unlock your files. With AlphaCrypt, there is, as of yet, no known method of safe removal.
Check the Kaspersky discussion boards for good and helpful info!
HiJackThis is a good option but, as I advised Peter, the first stop I'd do would be to run with Kaspersky's RescueDisk which is a suite of utilities that auto-boot from a memory stick, which will help you take back control of your computer and files.
A quick word of warning - if anyone gets infected with malware and searches online for a solution, don't just click willy-nilly on links promising to sort it for you. Do some research on everything that you might be tempted to install because, despite the amazing claims alongside links in Google, some things are not all they're cracked up to be!