Windows and Encryption Problems

Hi all! I am a newbie on encryption things, I usually work on mobile phones, and I need to ask you a question.
I have: A package file, encrypted, large about 20Mb; I don't know how it is encrypted, what algo is used etc., but I have almost all files contained in the package, fully decrypted. I need to know if it is possible to find the algo type, and maybe also the key, to extract and edit files in similar archives... And obviously how to! Can you do his? I can pay you if you for that... Or, can you suggest me another forum where I can ask?
Only serious replies please...
Regards