My MacBook has Bluetooth built in, but I'm not sure how to enable it and use it from within a Windows XP virtual machine. How does this work?
Just go to your VM's Settings -> USB Devices section and enable the "Apple Bluetooth USB Host Controller" to be connected to your VM. Then install drivers as normal on Windows.
Alternatively, click the small USB icon in the status bar of the VM when running in a window and then click on the Bluetooth item.