If you’re installing SQL Server 2008 on a machine that’s already got SQL Server 2005 Express Edition on it, then you might get the following error message:
‘Sql Server 2005 express tools are installed. To continue, remove the Sql Server 2005 express tools.’
This may mean that you’ve got SQL Server Management Studio Express installed. If so, you’ll need to uninstall it. This can be done by going to Control Panel, choosing ‘Add or Remove Programs’ and uninstalling the offending item.
Since you’re installing SQL Server 2008 (yay!), you won’t need it anymore anyway.
Jen Stirrup 
Agree with the unistallation but I have had times that I have had to go back and re-install 2005 BIDS, if you have to cover the transition from 2005 to 2008.