Do you have Steam installed in the Program Files folder?
Normally each save is a separate subfolder in the '..\Divine Divinity\savegames' folder.
Under Vista/7, depending on how the user accounts are set up, if you install the game to the Program Files path, Windows can create separate savegame folders for each user, and then map that folder to the game's savegame folder (so it will just see the saves for the relevant user). Changing the compatibility mode appears to have also change how Windows handles the save folder.

Check your '..\SteamApps\common\divine_divinity\savegames' folder, as well as the users folder:
C:\Users\ %account name% \AppData\Local\VirtualStore\Program Files\...
and move/copy your previous saves from the old location to the new one the game is using.