This will depends on the OS you have on your laptop or PC. But it should be almost the same with WinXP.

If it is Win7 it should be quite easy . You do it by going to the "Open Network and Sharing Center" which you can access by right clicking on your network logo on the left bottom corner or by going to Control Panel.

The solution is to create a network on your laptop in the "Open Network and Sharing Center --> Manage Wireless Network -->Add (new network) --> Create an ad hoc network. Fill out the information and save the network.
You need to allow these folders to be shared (in Open Network and Sharing Center --> Change advance sharing settings) by your new network to make it work. But you will be disconnected from your original wireless network if you set up your ad hoc network on the same adapter.

After connect your N900 to your new network.

You can in the same time access the internet on your phone (tethering) by this new created network ONLY IF you have a second adapter which will provide the internet by a bridge to the ad hoc network. SO YOU NEED TWO ADAPTERS: one providing the internet (a LAN network or a wireless USB) and another one for the ad hoc network.

If your original internet network is a LAN (or you have another network adapter like a wireless USB) you need to create a bridge between your original network and you new ad hoc network by going to Open Network and Sharing Center --> Change adapter settings. Right click on your original internet network --> Properties --> Sharing tab --> Check Allow other user to share .... and chose your ad hoc network adapter.

If anyone has an other solution, feel free to share it to everyone.

Cheers