This is from
www.bioware.com BG2 - known issues...
BG2 suddenly not knowing it is installed
If you have Norton WinDoctor or any other type of registry checking/cleaning utility it will delete the registry entries for BG2. While these entries are not wrong per se, they will be deleted by these kinds of utilities. Check to see if your system utility has an Undo feature.
You can either not run these utilities while BG2 is installed or you should run them in a manual mode that allows to you confirm actions like this.
If you are comfortable messing with your registry, you can manually re-enter the setting. Find the key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\App Paths\BG2Main.exe
and edit the (Default) entry and make it point to the BGMain.exe file including the full path. Also, create two new String values called "Install" and "Path" and enter the full path to your BG2 directory.
This is fixed in the BG2 Patch