You just need to copy/zip your saved games to another location (a USB drive, CD/DVD-R/RW and/or leave your existing drive in as secondary storage, at least temporarily).
If you have a digital distribution with limited installs, you will want to uninstall the game (while online) from the old drive and re-install on the new one, just to be safe. Changing a hard drive will probably not be enough to trigger another activation, and if you need more than the initial number allotted the process is pretty simple, but you'll need to re-install anyway, so might as well uninstall first.
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
If you did a lot of tweaking to your settings, you can back them up, as well (as above, but in the '..\Divinity 2\Profile' folder).
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).
Welcome to the forum.
