Help with Games for Windows Live

Hey ive just got fallout 3 for windows live and it automatically signs me in to my windows live id gamertag. i was wondering if there was anyway i could download mods or the expansions without paying via the marketplace. If so does this keep achievements (dont really mind if it doesnt) and also will it mean that my game save gets delete?

Ive also read about something called a Games For Windows Live Disabler, but didnt give nemore info than that.

You can play the game in offline mode, which doesn't sign you in, but you will have to locate your save game file and move them to a different folder.