Any time these newer versions of Windows loose their link and gain a new one, they will try to obtain a new IP.

Annoyingly, it seems to do this regardless of the link status. On numerous occasions, my DHCP server has gone down (due to power -- it's more sensitive to blinks than my PeeCee) and the Windows box almost immediately relinquishes its IP address, which is very annoying, since it can then no longer talk to the rest of the machines on my network.