I'm writing some licensing software & need to encrypt files on customers' computers so they can't read them, but be able to decrypt them at run time.

This all needs to be able to happen on Windows, Linux, and/or IRIX, and if I had my druthers, it would be able to work cross-platform across a network (but that's not necessary).

I've never really done any encryption, so I'm reading up on it now, but if anyone can offer a word or two of advice, that would save me some time.

any good books? websites? other forms of valuable information?

"The Code Book" by Simon Singh is the best source of information on encryption ever. It goes chronologically through the art of encryption. He shows you how to implement most ciphers and then how to crack them. I loved it and I'm sure you will too. Be sure to get the "real" one too, not the one for kids.