OK. I found some information. The first suggests uninstalling SQL Prompt. I tried it and that did the trick. The second suggest was to do a brute-force removal of the registry key: HKLM\SOFTWARE\Microsoft\Microsoft SQL Server\90. I didn’t get there. Install is running on the desktop now.
That seems a bit mean of Microsoft, requiring me to uninstall Red Gate’s SQL Prompt, especially since it’s such a great little tool. I wonder if I can install it on top of 2008? Time for some more research.