PrepareAD failed with error "Object reference not set to an instance of an object."

I uninstalled Exchange server 2007 manually (this is not recommanded by Microsoft). After that, I tried to prepare the schema. When I executed the "PrepareAD" I encountred this problem:
PrepareAD failed with error "Object reference not set to an instance of an object."
To solv this problem:
1- In Active Directory users and computers, make a search of an existing Exchange security group.
2- you have to delete them all.
3- Prepare AD another time