Hi,
Unfortunately, you cannot go back without either:
-Starting from scratch
or
-Starting from the last backup you have from the 2007 format
Unfortunately, 2008 uses a completely different file format system and coding (mySQL) which makes it impossible to go back to the old system (Access)
Have you done the latest update (Release C)? This release fixed some major performance issues with the 2008 version.
-Max