For Windows Vista/7: C:\Users\Username\AppData\Roaming\Ubisoft\Assassin's Creed 2
The config file's name should be Assassins2.ini. The above folders may be 'Hidden' under Windows. After finding this config file (.ini) right-click it and choose Open With.. and open the file with Notepad.
Once it's open, look for the section in the file that shows the resolution and compare it to the resolution of your PC. If the resolution in the config file is higher than what your PC can handle then you need to manually change it to a resolution your PC supports.