+1 to KalMaverick's question. If you didn't format/completely reinstalled Windows, then I would suggest that you do so after you've backed up all your data.
Fable 3 & Assassins Creed:Revelations are both DirectX 9 games actually, so it won't use DX10 or DX11 for them.
I would suggest that you install the latest version of the DirectX 9 redistributable:
http://www.microsoft.com/download/en/details.aspx?id=8109 * I'm not 100% sure that that is indeed the latest version
Lately I've also seen quite a lot of performance related issues due to certain graphics card drivers. Perhaps just download & install the latest drivers again off the AMD website. If you've already got the latest, then perhaps try using an earlier version.