To help you decode the last message, here's a clue:

If I'd wanted to spell my name in this code, it would have come out as: aaaaa ;-)

If you're interested in cryptography at all, the best book on the subject is The Code Book by Simon Singh.

A good site for introducing cryptography (aimed at kids but I like playing with it) is:

If you find any of it confusing, I'd be happy to pass on any tips I can.

B-)