You have two problems here.
Firstly Biztalk 2004 is not supported on SQL 2005, only SQL 2000.
Secondly, I don’t believe you can install the BRE in standalone mode with Biztalk 2004. You must install Biztalk as well.
You might like to try Biztalk 2006 beta 2, this will use SQL 2005, but you will probably need the Standard Edition as a minimum.
I think with 2006 you can also install the BRE standalone.