it's possible that? i've readed on some sites instructions for linux but i didn't succeeded to share my n900 wi-fi connection(router-n900 n900-via usb to pc) to an old pc that hasn't wi-fi .i know the buy the wi-fi card option , I'll buy it , but dammm it's linux , maybe i learn something.

use qt mobilehotspot cool program to wifi and usb tethering

Doesn't start, i get a fixing problems pop-up and closes

I used to do something like this for a DIY wireless bridge. Look up iptables nat routing and i think you will find what you need, just bridge the two network interfaces and you will be set if NATing is not a problem. Once you have it figured out write a startup and kill script, there is even a script launcher app in the repos.

you may want to use the search:
http://wiki.maemo.org/N900_USB_router

I've used this (with Linux as client), and it works perfectly fine.

I find MobileHotspot works fine (non-Qt version)

mobile hotspot can't share wi-fi connection via usb

first install kernel power and kernel power settings. follow program guide install driver in windows. i used it everyday