Linking IP address to PC's

I would not see that happening on the PC itself. either it has a fixed IP address and then it would have a very hard time to send the email when plugged on a different subnet or it is a dynamic IP address (DHCP) and then it would not really know whether the change of IP is due to a physical location change or a lease change. If in DHCP checking for the default gateway via the output of ipconfig or netstat should detect the change of subnet.

My best choice would rather check that on the network itself, having the manageable switches report new mac addresses and then compare that to some sort of "expected mac addresses" table/list. But this will not track you computer on a unmanaged (by you) network.

on second thought you may devise some service or startup/scheduled task that will report every now and then its identity (computer name) and location (current IP and default gateway) to a server. The server will then compare what's to be expected for this computer and then issue the warning.
on quick and easy (a bit dirty maybe) I'd devise a wsh script to issue (silently) an http request with GET arguments taken from the client computer (hostname, currentIP, current default gateway, maybe logged on username)

but then we are going toward automatic inventory system, so you may want to upscale to solutions like insight manager or other configuration management tools.

Dynamic IP Monitor has the functionality you're talking about.
http://www.guldendev.com/?display=dynamicipmonitor

Might be exactly what you need.
-Matt