Hi,

In order to get internet connectivity from this "ad-hoc" connection - you should allow what is called "Internet Connection Sharing" on your windows PC.
This is supported on windows since WIN98-SE and on, and has gotten to be much simpler since windows XP and on.

To allow sharing of your internet connection :
1. Locate the network interface which actually connects you to the internet. Lets say it is "Network connection 1"
2. Right click it in the "Manage interfaces / adapters" or however it is called (depends on what windows you are using), and enable the "Share this connection with other..."
3. If you have more then 2 network interfaces, will be asked to indicate to which interface you would like to share this connection, choose your "Wireless interface". HIt ok.
4. Connect your PC to the internet, establish the ad-hoc network between you n900 and the PC, make sure DHCP is enabled on your n900, (if you don't know what it is, then it is probably already in DHCP mode, as it is default).

You should now have internet, both on PC and n900, providing the PC is on.