I would think you're out of luck with Windows 2000
XP should be okay
Google returned these, looks like the SQL Server 2008 Express won't run on 2000
http://technet.microsoft.com/en-us/library/ms143506.aspx
http://msdn.microsoft.com/en-us/library/ms131002.aspx
As for Client Tools installer, as far as I know, you have to download the ISO and use that to install the Client Tools only
But good thing is, the ISO also contains Upgrade Advisor or System Requirement Check to ensure you can install SQL 2008