am using curently Gnome and Nautilus which
works very slow . So I download and installed rox filer
from tgz files. I type rox and it works but how to replace nautilus which is a defualt file manager.
Any idea???

I think that is a very good question that you may find some trouble getting answered, here is my humble suggestion...

Use ROX with something not so "resource hungry". Something like sawfish, or flux or whatever you'd like, but not Gnome or KDE. This is just my oppinion. If you don't get an answer from any of these wonderful people soon on how to replace nautilus, you may want to read all you can from the Rox website, and if you don't find the answer, write an email directly to them, or post this question in thier SF forum.

Cool