You have an ESXi server running 4.0 and you want to update to 4.1
Note: This procedure is carried out on a stand along ESXi 4.0 host, if you have 4.0 ESXi servers managed by Virtual Center you MUST update your Virtual Center to version 4.1 FIRST. If you don’t you cannot connect to the updated 4.1 ESXi host post upgrade, if you try you will see the following error,
Error: HA agent disabled on {hostname}
And for Virtual Center 4.1 the host server needs to be x64 bit (at time of writing Server 2008 R2 is NOT supported).