I'm playing the game on Windows XP using my steam ID and I've unlocked 2 achievements already but my girlfriend is playing it as well on Windows 7 using my steam ID BUT with a different save game so if she unlocks any achivements will it interfere with the achievements I've already unlocked? As in if she keeps playing it using my ID but with a different savegame if she unlocks any achivements will the same achievements then be unlocked for the savegame I've got as well meaning I won't be able to unlock them myself?

Just asking because I'd prefer for that not to be the case but I've a funny feeling because she's playing the game using my steam ID even though it's on a different version of Windows and using a completely different savegame that any achievements she unlocks will be unlocked for me as well?

The in-game achievements, called challenges, that you can look at from within your Pipboy are definitely on a per-character basis. The Steam achievements viewable only through Steam are on a per steam account basis.

Thanks for replying,

That's what I thought re: The Steam Achievements being on a per account basis, so basically if my other half does one of the achievements before me then I can't get it (but technically I already have lol) thanks again for clarifying.