- Backup your VirtualCenter database. VMware recommends detaching the database and copying it somewhere safe.
- Grant the System DSN user of the VirtualCenter Database db_owner privileges on the MSDB database as well as the VirtualCenter database.
- Ensure that your ODBC System DSN is using the proper driver. You must have a SQL Server driver if your database is SQL 2000, and SQL Native Client driver if using SQL 2005.
- If your ESX Server host is in a DRS or VMware High Availability (HA) Cluster remove it from the cluster so no virtual machines migrate to the ESX Server host.
- If possible VMotion all virtual machines off the host to be upgraded. If not, power them down gracefully and power off the ESX Server host.
- Log in to your VirtualCenter server with a local Administrator account to run the installation, do not use a domain administrator.
- Perform the upgrade to VirtualCenter to 2.5 and ensure all your data is visible in VirtualCenter 2.5 after the upgrade.
- Ensure no processes are running that conflict with the ports that VirtualCenter uses, such as IIS.
- If there is a SAN connected to your ESX Server host detach the SAN before continuing with the upgrade.
- Confirm that all the virtual machines are now migrated from the ESX Server host or powered down, and that ESX Server host is no longer part of an VMware HA or DRS cluster.
- Download the newest version of the ESX operating system ISO image and burn it to CD.
- Place the CD in the CD-ROM drive of the host and boot from the CD.
- Install ESX Server 3.5 with a fresh install or upgrade.
You can find the VMware KB 1003889 article here.
Posted by Gabriel Maciel