Want docs to use Office 2004 apps by default

Version: 2008
Operating System: Mac OS X 10.4 (Tiger)
Processor: Power PC

I have Office 2008 and 2004 installed, and both seem to work fine.
However, my computer is older so I would like to continue to use Office
2004 by default, and Office 2008 selectively.

The problem is, setting a doc to open with the 2004 version never
'sticks'. Even if I use Finder->Get Info, and set a .doc or .xls file to
open with an Office 2004 app, and even click 'Change All' so all apps do
this, double-clicking always results in the 2008 app launching.

Is it possible to change this?

Thanks!