With vCenter Server 4.1 in place you should be able to login to vCenter Server (using the vSphere Client) using an account with administrative permissions, i.e. a Domain Admin or a user who is a member of the vCenter Server's local "Administrators" group. If you can do this, the next step is to create a global group in your domain with the name "ESX Admins" and add your user account to this group. After this, configure Active Directory authentication for the ESXi host using the vSphere Client while connected to the vCenter Server (select the ESXi host in the inventory, click the Configuration tab and select "Authentication Services" from the Software pane). Once done you should now be able to use the vSphere Client to connect directly to the ESXi host using your domain account, which allows you to change the root password from the "Local Users & Groups" tab.
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