I'm looking for an easy way to retrieve the WAN IP of a computer
(behind a router so ipconfig won't work) and then email it once in
awhile to a given email address. Can I easily make a script or service
or whatever that will run on a Windows XP Pro computer that queries
www.whatismyip.com for the IP
address and then emails it? I know very little about web programming
but if its an easy task I'm willing to learn enough to write it.

The answer is probably "no, it wouldn't be easy for you."  For a more
definitive answer, you should specify how smart you are, what you know
that may be relevant such as math or network administration, and how
much time you would be willing to spend on the task before you would
consider it not easy.

If you are handy with a scripting language you could examine the
output of a call to whatismyip.com:

     lynx --source whatismyip.com

The output to this call will contain the IP address.  You would just
have to parse the text returned to find it

I see no other reason for this question than that you probably run
some kind of server which you want to reach, even if for some reason
you get a new IP assigned by your ISP.
In that case, the mailing part is not really important, just that your
PC is always reachable with a simple name, for instance
makenewname.is-a-geek.com or pcisreachableat.no-ip.com, which resolves
to this IP number. Take a look at http://www.dyndns.org or
http://www.no-ip.com, register for a free name, and then install a
tiny client running on the server, which communicates with the name
provider whenever a change happens, which will distribute it quite
quickly over all the world's DNS servers.