Hello and welcome from another new forum member!
Like you, I'm switching from Sony to Pentax. I have an A100 too (which I'll probably keep after my K7 arrives), maybe I can help.
First off, what kind of adapter did you get for the A100, chipped or non-chipped?
If it's non-chipped, the blurriness might be caused by SSS, because the camera does not know what FL the lens has. AFAIK, the 'default' focal length assumed by the camera is 50mm, so it shouldn't be a problem (in theory), but it might be worth it to take a few shots with SSS turned off, see if things improve.
One other thing: my non-chipped M42-to-A-mount adapter needed a bit of polishing on the side facing the lens, otherwise my lenses wouldn't focus to infinity.
On the other hand, if you're shooting wide open, the blurriness might be caused by a slight back/front focusing, I know it can be difficult to judge focus accuracy in the tiny viewfinder the A100 has (A700 is much better in this regard).
EDIT- I assumed you were talking about an M42 lens, and of an adapter for its mount; if that's not the case, I apologize - I'm afraid I haven't used K-mount lenses on Sony bodies, so my thoughts above are most probably useless