In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
It is not Larian that marked the app data folder hidden, and set Windows Explorer not to show hidden or system files by default, but Microsoft.
It is also Microsoft who is responsible for setting the standard for saves being made in the user folder structure (so that different users on the same computer can keep separate sets of saves / options / etc). In fact, even with older games that store saves in a subfolder of the main game folder structure (like DD and BD), Windows Vista/7 will still create the savegames folder in the user folder, and map that to the location the game expects to find when it is run.
While I certainly sympathize with you for loosing the saves, you should always make a full backup before wiping a partition to do a clean install of Windows. There is always going to be some data not in a standard location, or a program / dll / serial number / password you can not find later, or something else that can be missed in a selective / partial backup.
