If you installed the 2008 with Default instance name, then you must have specified a name for the 2005 instance. Also, if you run the SQL Server Configuration Manager from the 2008 start menu, you should see two instances (or one, at least) and their statuses. If not, I hope you didn't over-wrote the 2008 instance with 2005. I doubt that's possible, though.
If Database Services were installed successfully during both installations, you should have two instances. If not, then the database services module must have failed.
I'm using 2005, 2008, 2008 R2 n my x64 Win 7 machine. I did installed the express editions and respected the version order for installing. But I call my 2005 instance
SQLExpress2005, 2008 instance
SQLExpress2008, and 2008 R2
And if you do find any instance, connect to it using 2008 Management studio and check the instance version against this: http://sqlsecurity.com/FAQs/SQLServerVersionDatabase/tabid/63/Default.aspx