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

Exchangeserver.msi failed with error code 1603

I was in process of installing Exchange server 2007 SP3 in a client plateform, and the famous error code 1603 appeared. After taking a deep look at the exhangesetuplog I detected the following:

"exchangeserver.msi failed. Fatal error during installation. Error code is 1603. Last error reported by the MSI package is 'Could not open key: UNKNOWN\Components\2E016F08F75B6F848A40D36348D1134D\AE1D439464EB1B8488741FFA028E291C.
Verify that you have sufficient access to that key, or contact your support personnel.'. "

I copied the parent Key and I searched it in the regedit, I found that as an Administrator, I hadn't the required permission to acces the child key. Strange!!!
I solved this problem by this way:
1- Editing the permission in the parent key, and editing assigning the administrator account the ownership of the key
2- Expand the permission to the child key
3- Editing the permission of the child key and assigning the administrator account the ownership of the key.
3- Restarting the server
4- Restarting the installation process.

Afer doing this tasks, the same problem appear with others keys, so you have to repeat this tasks and the installation will be successful.
This problem may affect Exchange server 2010 and Exchange 2010 SP1 installation.

Good luck