How do I make OpenOffice.org portable be my default document viewer on Windows?
I use portable apps a lot, and I think the best way to make file associations (even permanently) is by using
Portable File Associator, which is programmed using AutoIt (yeah!). Unfortunately, I cannot find a working download link for it. I do plan to make my own modifications to it soon. I can give you a direct download link from my computer if you are interested in using it.
An alternative is eXpresso, which is made using AutoHotKey (boo!). I do not like the way this one works that much, as it intercepts clicks rather than making registry entries in HKCU.