how to use structured markup tools

[Sean McIlroy]

I'm dealing with XML files in which there are lots of tags of the
following form: <a><b>x</b><c>y</c></a> (all of these letters are being
used as 'metalinguistic variables') Not all of the tags in the file are
of that form, but that's the only type of tag I'm interested in. (For
the insatiably curious, I'm talking about a conversation log from MSN
Messenger.) What I need to do is to pull out all the x's and y's in a
form I can use. In other words, from...

.
.
<a><b>x1</b><c>y1</c></a>
.
.
<a><b>x2</b><c>y2</c></a>
.
.
<a><b>x3</b><c>y3</c></a>
.
.

...I would like to produce, for example,...

[ (x1,y1), (x2,y2), (x3,y3) ]

Now, I'm aware that there are extensive libraries for dealing with
marked-up text, but here's the thing: I think I have a reasonable
understanding of python, but I use it in a lisplike way, and in
particular I only know the rudiments of how classes work. So here's
what I'm asking for:

Can anybody give me a rough idea how to come to grips with the problem
described above? Or even (dare to dream) example code? Any help will be
very much appreciated.

Peace,
STM