1st, take it off the internet.
Consider if you really needed any of the files. If not, reinstall windows and be more conscious of security in the future.
If you you really need some of the files, you can try to investigate which trojan it was and if the files can be recovered. It's possible the maker of the trojan didn't really know what he was doing.
If it's a proper trojan, the files are encrypted and only buying the key might get his files back. If the criminal keeps his word.
Also if it didn't convert everything to .vvv files yet, he should not start Windows at all, but boot a Linux livecd and recover any undamaged files to a usb stick.