Strange not to see a windows version shown on the System information image. I thought any installed version of windows came up on this page.
Can this information be hidden somewhere else?
I did not see Windows Update KB935509 mentioned.!!! (This software update is a prerequisite for Windows Vista Service Pack 1 (SP1). This update helps improve reliability when you install or remove Windows Vista SP1.) See the following link:

A software update is available for versions of Windows Vista that include the Windows BitLocker Drive Encryption feature

•Windows Vista Enterprise •Windows Vista UltimateThis update is necessary to install and to uninstall Windows Vista SP1 successfully on these editions.