When installing SQL Express 2008, the default instance name is SQLEXPRESS.
Unfortunately, this is the same default as SQL Express 2005. That means if you have SQL 2005 Express installed, you probably already have an SQLEXPRESS instance, which is a conflict.
The trick is, when installing SQL Express 2008, to change the name of the default instance to something different. Mine is named SQLEXPRESS2008.
I have SQL 2005 Express, SQL 2005 Developer, SQL 2008 Express, and SQL 2008 Developer on my main machine, and they all work.