Help with automatic releasing of IP address on WIFI adapater

This is a bit different, but here goes:

I have a scenario with a Windows XP laptop that I want to use the WIFI for primary, the PPP adapater for backup, and the issue I’m having is that as long as the IP address stays bound to the WIFI adapter, the default route doesn’t switch to the PPP adapater, as the default route for the WIFI adapater is still in the table. Once I release the IP from the WIFI adapater, it switches correctly.

Does anyone know of a way to get the WIFI adapater to automatically release the DHCP address when it loses communication with the access point?

Thanks!

Kim