You could try a file recovery program such as
FreeUndelete, but it is likely the previous save was overwritten. On Vista/7 there could possibly be a shadow copy of a previous quicksave (IIRC you need a pro version to officially restore shadow copies, but it is enabled in the home version by default [at least in Vista], and there is a third party program to view/restore files - don't recall the name).
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders.
