An update has been released for Assassin's Creed via steam, and will be downloaded automatically when the steam client is restarted.

Assassin's Creed

• Fixed a rare crash while riding the horse in Kingdom
• Fixed a corruption of Altair's robe on certain graphics hardware
• Cursor is now centered when accessing the Map
• Fixed a few problems with Alt-Tab
• Fixed a graphical bug in the final fight
• Fixed a few graphical problems with dead bodies
• Fixed pixellation with post-FX enabled on certain graphics hardware
• Fixed a small bug in the DNA Menu that would cause the image to disappear if the arrow was clicked rapidly
• Fixed some graphical corruption in Present Room with low Level Of Detail
• Character input is now cancelled if the controller is unplugged while moving
• Added support for x64 versions of Windows
• Fixed broken post-effects on DirectX 10.1 enabled cardd