If you are running Peachtree 2007 and any older version of Peachtree on the same machine, you will need to end the process "w3dbsmgr.exe" after running Peachtree 2007 before you open a prior version.

To do this, go to your Task Manager (press control-alt-delete simultaneously), click the Processes tab, right click on the process "w3dbsmgr.exe" and choose the option to end process tree. You should then be able to open an earlier version of Peachtree with no problems.