U need 2 USB Bluetooth dongles for that.One will be on ur desktop and d other on ur laptop.Create a Personal Area Network and share the internet connection.Disadvantage is that ur desktop will be on when u'll surf on the laptop.
If ur using XP Sp2den go to CTRL Panel -> Network Connections.Dere u'll find
"Bluetooth Network Connection".use it to pair ur desktop and ur laptop and share the internet connection.