Install SQL Server 2005/BCM 2007

I have Windows XP Pro-upgraded from Office Basic 2003 to Office 2007 Pro. All OK but cannot get SQL Server Express to install and therefore also Business Contact Manager won’t install. (Off upgrade disc)Tried downloading and installing SQL went OK but when I put in the upgrade disc doesn’t recognize installation and tries again. Have removed all traces of installation- any ideas what I am doing wrong?
Any help much appreciated.