Instance GUID is improperly specified

Recently I was trying to install vCenter when I received the error "Instance GUID is improperly specified". This was a brand new Windows 2008 Server and a fresh install of vCenter, but the problem was that the install had failed a few times already due to other issues and rolled back. It seems that the rollback process does not completely clean up the files it creates, because one such file called 'instance.cfg' is created in c:\Program Data\VMware\VMware VirtualCenter and in it there is a line with the GUID. So to resolve this, simply delete that file and it will be recreated upon installation.

Error 25003. Setup failed to create the vCenter Server repository

http://www.vmware.com/support/vsphere4/doc/vsp_esxi40_vc40_rel_notes.html

vCenter Server installation on Windows Server 2008 with a remote SQL Server database fails in some circumstances
If you install vCenter Server on Windows 2008, using a remote SQL Server database with Windows authentication for SQL Server, and a domain user for the DSN that is different from the vCenter Server system login, the installation does not proceed and the installer displays the following error message:
25003.Setup failed to create the vCenter repository.
Workaround: In these circumstances, use the same login credentials for vCenter Server and for the SQL Server DSN.