If you upgrade OS X to Snow Leopard, and Word 2004 starts locking up, freezing, not responding when you try to open a document, forcing you to force quit, then it's probably a font cache issue.

The easy way to clear the cache is to reboot with the "Shift" key held down. This does a "safe mode" boot. Then log in, and verify you can open your document. If it works, restart into normal mode and it should just keep working.

This probably also works: Open a terminal, run sudo atsutil databases -remove, and then reboot.

If this doesn't work, you might need to open the Font Book application and do Edit -> Resolve Duplicates. But I don't think that step was actually necessary.

Hth. It's alluded to around the internet, but I thought I'd put our experience out clearly.