ME1 is totally playable with Win8. Just finished three complete playthroughs on my Win8 Pro/64-bit rig to get the Insanity Achievement. Again.
Two suggestions:
1) Make sure the EA/Origin client (if you launch the game that way) is set to "Run as Administrator", and;
2) Follow the instructions found in this topic: [Information] Mass Effect 1 and Windows 7 - You need patch 1.02 to play, which is pinned to the top of this forum. (Applies to Win8 as well.)
EDIT: Hey MLadyD7, thanks for linking to my previous solution. 😕mileywink:
I just updated it with some additional info I've learned since then, (already included in this post.)