I think the issue you're facing is your graphics card. I left Nvidia after my 7950 GT OC issues. Good GOD, their drivers sucked, the card was unstable as all hell, even when underclocked.
I doubt it's the graphics card alone. I think it lies somewhere in having an early dual core amd64 (2GB ram btw).
Every other game (Crysis, COD4, MS flight, Il2, video editing, whatever) runs fine, so did AHII 2.13
And without compatibility mode, I don't get any stutter - but sooner or later a spinning clipboard or a almost certain lockup when minimzing AH (which I absolutely HAVE to)
And before someone asks: Yes, I know about the optimzer, but no combination of optimizer and cpu driver versions did ever allowed me to run AH without compatibility mode. (Tried them all on a fresh XP install)
*sigh*
Currently I just could kick myself for havin invested all my money in this 9600GT in expectance of AH 2.14