I decided to upgrade to Vista Premium, after backing up I inserted the Vista upgrade disk. I came to a window which said something like "You cannot upgrade winxpro to Vista premium, you must upgrade to Vista Business" (or something). The upgrade option was greyed out, but the "Install" option was available. I took a chance and clicked install. Vista loaded very quickly without any problems. So what happened to winxppro, as I was not advised that everything would be overwritten, or removed? I now have Vista Home Premium installed as I wanted. Has everything been removed from the HD that I had in "My Documents"? not that I mind.
Thank you both, I have indeed found a folder called windows old with the contents of winxppro in it. I wasn't too concerned about the loss of anything, just interested because the install was so clean, fast and uncomplicated. I've looked in "System" and it tells me that it is an upgrade, not the full version.