When installing Office 2003 over the top of pc's that have Office XP, the install just about finishes then begins to roll back the changes and finally quits with an "Installation Ended Prematurely because of an error" message. That's not very much to go on!
I'm using a custom build of Microsoft Office 2003 created using the Office Resource Kit. I've created custom builds in the past for Office 2000 and have never had any issues. I'm really not sure the fact that it is a custom build has anything to do with the problem quite frankly, but I thought I needed to mention it. The reason I say so is because the custom build installs perfectly on clean machines without Office on them already.
I've searched the internet and have found a number of interesting links with the exact error, but each points to a situation that isn't the same.