(a FAQ?) window/door replacement information?

Hi all

I'm guessing this is a frequently asked question; sorry if it's a repeat. I looked back a few pages of posts, but didn't find anything addressing my question:

I'm looking to replace two external doors, one single-width window, and two double-width windows. These all leak quite a bit and aren't very attractive.

So far, I've found varying bits of information about this process, and wide differences in prices. I'm feeling a bit overwhelmed right now. What should I be looking for? Is there a "beginner's guide to contracting window and door work" somewhere that can give me a starting point?

Thanks

 

Goggle: "how to buy replacement windows"
http://www.google.com/search?as_q=&n...h=&safe=images

pretty god advice here (from above search)
http://www.decorate-redecorate.com/windows.html