According to the vCenter Installation guide http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-installation-setup-guide.pdf - SQL 2005 is not supported only SQL 2008 Epress is supported - check page 162
You will have to back up the database and restore it to one of the supported databases - the upgrade guide should have the steps on how to switch database engines - http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-upgrade-guide.pdf
Thanks for your reply....If I run the installer, I get
"The installation detects the current vCenter server database and the used ODBC driver. Check whether the correct database is displayed to avoid upgrading the incorrect database"
I have also read on other blogs that you can just upgrade the current VC.
the upgrade guide on pg 26 suggests we can do a in place upgrade of VC4.1.
Page 32 also suggests that After you upgrade to a database server that is supported by vCenter Server, you can install or upgrade to vCenter Server.
so this is confusing. Some sites suggests you can do a inplace upgrade while the guide says you can't use 2005 express edition.
The only MSSQL Express version supported by vSphere5 is 2008 but in docs is written:
You can perform a fresh installation of vCenter Server 5.0.
looks like you have to do fresh installation not upgrade