Dear Sandeep,
Many thanks for your advice. I have followed your advice to restart both SQL Server & VMware VC Service but still get the same warning message. As the rollup is not related to SQL Server Agent, I guess that the SQL Server 2005 Express may be corrupted during upgrade.
In this way, I install another instance of SQL Server 2005 Express & reinstall vCenter Server. The warning message "Performance statistics rollup for Past Day to Past Week is not ocurring in the database" in vCenter Service Status is gone.
I just wonder whether it is related to "No performance data" issue.
Thanks again
Message was edited by: TonyJK