[andrewas]

first up, you say you cant install, but can you un-install. if you can then your probelms should go away. elswise read on:

you mean "swapped" OS, right? in that case then it should work. can you un-install BG? if not then at somepoint you killed your BG registry entries. on win 2000 and win XP some utilities do that and you need a patch from bioware.

you cant really repair the registry manually, youd need someone to go through theirs and get all the BG keys, and i think some of them change to reflect your hardware.

if you dont have 2000/XP then try to remove all BG keys from your registry (not all are important, but try and get them all anyway) and delete SoA. save your saves of course. then re-install from scratch.