I found the answer

I had to set permissions in the registry HKEY...ROOT. I had just one entry -
Everyone. I added one for each of the accounts in Documents/Settings -
Administrator, etc.
Rebooted, installed, and all is well!!!!!
The msg on the install failure was "Access Denied"
"JustUs" wrote:
> Just upgraded from XP Home to XP Pro. Now the little yellow update icon in
> the systray will not go away and I can't untick - cause Win Installer 3.1
> fails to install. Have read lots of the forum posts, but .... no joy.
> Suggestions.