Are you only using quicksaves?
Though rare, a couple people have had corrupt version numbers written into one of the files in the save folder. Hex editing the quickinfo.000 file to fix the version number allowed the saves to be loaded.
Savegames are saved as VR.A and wont open Make a copy of your most recent save folder. If you are not comfortable hex editing the quickinfo.000 file to fix the version number, it should be ok to copy that file from the working save into the copy of the non-working save.
As long as the rest of the save was written properly, the save should load once the version number is fixed (a new game folder should contain 22 files, after going to either Verdistis or the Dark Forest it should have 27, after going to both it should have 32, and then 37 or 42 after getting to other areas late in the game).
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).
C:\Users\ %account name% \AppData\Local\VirtualStore\Program Files\Divine Divinity\savegames
