Windows XP: C:\Documents and Settings\Username\Application Data\Ubisoft\Assassin's Creed
Windows Vista: C:\Users\Username\AppData\Roaming\Ubisoft\Assassin's Creed
Open this file with notepad and find the line:
Change it to:
Save the file.
This variable controls whether Assassin's Creed starts up in fullscreen (=1) or windowed (=0) mode. Fullscreen mode is always recommended for optimal performance and minimal problems, especially with Windows Vista where fullscreen applications automatically disable the desktop and thus improve performance. However, if you find the game starting up in windowed mode even if you don't want it to, make sure this option is set =1 and if that doesn't work, press ALT+Enter to force the game to switch to fullscreen mode.