Microsoft Office 2004 reinstall?
Make a good backup in case anything goes wrong, but if you install Mac OS X Snow Leopard using the default options, applications previously installed will remain in place with licenses intact.

One particular issue facing Microsoft Office 2004 under Snow Leopard is that Office 2004 is a PPC app, and thus requires Rosetta to run under Snow Leopard. Rosetta is not included by default in a Snow Leopard install, but rather needs to be included in the list of checkboxed items when you get the list of what components to install.

For more details about the Snow Leopard install process, you can read this article from Macworld.