Hello,
Edit AT.ini (Win XP C:\Documents and Settings\All Users\Application Data\Bentley\Plant V8i\CFG and Win7 C:\ProgramData\Bentley\Plant V8i\CFG
Change CADAPP variable in all sections so it would point to corect AutoCAD version.
From CADAPP=C:\Program Files\Autodesk\AutoCAD 2012\acad.exe to CADAPP=C:\Program Files\Autodesk\AutoCAD 2008\acad.exe
I hope this will help.
Regards,
Artiom