Try this:
I was using this method when I was in a hotel that does not allow Wifi connection only LAN cable. And I need to make a phone call using my skype from n900 and I want to do it from my bed.

This is how I do it:

From Windows XP
1. Share Your ICS thru LAN
1.1 Open the “Ethernet” LAN properties and allow ICS
2. Open the Wifi Properties and do as below:
2.1 Under Association: (You can change the encryption key to something else)
2.1.1 Network_Name: MyHomeNetwork
2.1.2 Network Authentication: Open
2.1.3 Data Encryption: WEP
2.1.4 Network Key: a1b1c1d1e1
2.1.5 Confirm Network Key: a1b1c1d1e1
2.1.6 Key Index (advanced): 1
2.1.7 The key is provided for me automatically: Unchecked
2.1.8 This is a computer to computer (adhoc) network; wireless access points are not used: CHECKED
2.1.9 Press OK
2.1.10 We are done with Computer

3. Configure your Nokia N900 to use the MyHomeNetwork using the a1b1c1d1e1 key
I assume you know how to do this.

4. Then try to connect from your N900 to make a connection to MyHomeNetwork
5. Go back to your XP, and try to make a connection to MyHomeNetwork also
Once you see “Connected”. You are done.

6. Try to surf the web using N900, you should be able to connect to the internet now.


Note: Once works, you can change WEP to WPA2.