The Assassin’s Creed save games can be found in the following locations:
C:\Documents and Settings\<username>\Application Data\Ubisoft\Assassin's Creed\Saved Games
C:\Users\<username>\AppData\Roaming\Ubisoft\Assassin's Creed\Saved Games
These are the default locations, if they are not in the respective location you can try searching for assassin.ini as they should be in the same folder.
I recently started to play Assassin’s Creed on the PC again after a long break and I started getting graphical corruption such as in the picture below, floors and walls were disappearing completely, it was like a wall hack had been enabled. Checking online I found out the it is a conflict with Xfire and can easily be solved by disabling the in-game xfire just for Assassin’s Creed . This can be done by opening ‘Tools > Options’ and then click the ‘Games’ tab, select ‘Assassin’s Creed’ under the ‘Installed Games’ then check ‘Disable Xfire In-Game’ and finally click ‘OK’, this will save the setting for future use.