Your wifi won't help you in this case.
What you need is Nokia's PC Suite software and the USB cable that came with your Nokia 5800 to attach to the PC. This is "tethering" and allows you to access the internet through your mobile.
If you want a wireless connection between the PC and Nokia, then your PC needs to be Bluetooth enabled, but that will result in a slower connection than USB.
i have a nokia 5800 which is wifi enabled and a laptop(windows vista) with Ethernet internet connection and it is wifi enabled too.
how can i share my laptop internet with my cellphone using wifi??