Realtek is always updating its drivers, since its audio devices are used by a whole lot of motherboards and other things, and the newest one showed up just a short while ago.
This once, the company didn't offer any big change log, not in the strictest sense anyway, though it is a given that some fixes and refinements are provided where needed.
What Realtek did do is supply a nice list of all supported controllers, for Windows XP, Windows 2000/2003, Windows Vista and Windows 7.
Indeed, unlike other software packages, which don't bother with any Windows OS prior to XP anymore, Realtek's new 2.66 driver does support Windows 2000 and 2003.
Follow either of the links below for the appropriate version.
Realtek HD Audio Driver 2.66 for 2000/2003/XP
Realtek HD Audio Driver 2.66 for Vista/Windows 7
Realtek HD Audio Driver 2.66 for Vista64/Windows 7 x64
Download Realtek HD Audio Driver 2.66 for XP/Vista/Windows 7
People should always make sure to have the latest drivers installed
Halo: The Master Chief Collection 20 GB Day One Patch Eliminates Need for Second Disc, Developer Claims
... so hot right now