Is it possible to have SQL Server 2005 express and
SQL Server 2008 express installed and use both ?
No and yes.
You can have both of them installed.
But the installation programs does not like it - you
need to remove a 2005 registry entry while you install
2008 and then put it back in after the installation.
I have never tried running both at the same time, but
if there are no port conflicts, then that should work
as well.
Arne