The multiprocessor hal.dll is probably the root cause.

"Tom Porterfield" <tpporter@mvps.org> wrote in message
news:uNndJEXRHHA.4476@TK2MSFTNGP05.phx.gbl...

> There are features in XP Pro that are not in Vista Home Premium. Things
> such as support for two processors, EFS and others that prevent a clean
> way to do an in place upgrade from XP Pro to Vista Home Premium.
>