can use this program instead of XSI??

would be great because this is a portable version. so i can model also if i'm not a t home
http://www.softwareload.de/shop/k-3d-portable

Why not use Blender? It's a lot more versatile and definetely works for props in SWBF.

is this portable?? where can i download??

It just works straight out of the .zip file. (the windows installer is just for integrating shortcuts and file associations).
Blender 2.49:
http://www.blender.org/download/get-blender/
Blender 2.56:
http://www.blender.org/download/get-256-beta/

2.49 has an integrated VRML97 importer and exporter (the format SWBFviewer can save out of a .msh file), but 2.56 is waaaaay better to work with, you can also work with 2.56 and export through 2.49 or any other program that takes .obj files.