If the keylogger encrypted the data, it will likely have the encryption key stored locally. Assuming a symmetric algorithm, if you can find the key, you can decrypt the file. If the logger is using an asymmetric encryption algorithm, well, find the encryption key tells you nothing. I'd bet on the encryption being symmetric, though, because asymmetric takes a lot more CPU.
If you can, watch what system activity happens when the keylogger is started. On windows, for example, monitor the registry reads, filesystem reads, and such. The key may be stored inside the program file, and if so, then you'll have an interesting challenge to figure out the key. If you want to find the attacker, let the logger start up and watch the network traffic. I'd bet no matter how the program phones home, that phone will be some sort of anonymous drop. But you never know, you might get lucky!