I don't quite understand what you're trying to do here.
Do you have a wired connection from your ADSL modem to your laptop and then want to use the laptop's WiFi to connect the N91 to the internet?
If so, you'll need to bridge the two network connections on your laptop (I think it's quite easy in the Windows Networking Wizard) and hope that the N91 supports "ad hoc" WiFi networks. If you look on the N91's support page on Nokia's website, there's some easy to follow guides on how to set-up WiFi on your phone.
I don't think it's possible to use the Bluetooth connection without some serious hacking.
Alternatively, you could buy an ADSL wireless router and connect your N91 directly to that. It'll cost you £50, but it will make everything a lot easier.