Windowless Applications with Transparency

[john mckerrell fastsearch com]

Hi,
   I need to write a application that will give me a "desktop buddy", a
picture of a little cartoon character on my desktop that will animate,
interact with the user, that sort of thing. I think I'd like to use
GTK(2) because it's freely available, and for the following reasons. The
primary platform for this is Windows, but I personally use Linux 99% of
the time so I'd like it to work on both platforms, and being able to
develop it on Linux would be handy.
   So, am I simply barking up the wrong tree, is this not the kind of
thing you can/would do using GTK? Would the actual drawing of the
character be done by something lower level, but dialogs could be GTK?
   If I have got it wrong, can anyone on here point me in the right
direction for what I need?
   If I haven't, has anyone got any tips for where I should start, or
know of any existing open source programs that I could take a look at.
Thanks for any help you can offer, sorry if this proves to be off-topic.
Regards,
John McKerrell