WIN32 API and Linux... *shudder

Hi, I want to write a cross platform program for windows and Linux, I love Linux but am stuck with windows on such a common basis, and so I want to make my program use the WIN32 API so that I can make toolboxes and file searching and all that good nonsense, however I have no idea how WIN32 programming would be transcribed into Linux programming. Is there an easy way to do this? Or am I going to have to use Wine or something like that to run my program without extensive reprogramming of the GUI functions for Linux?

E.G. is there a call for Linux equal to the main WinProc function for WIN32 programming? I probably sound like a newbie when it comes to Linux programming and that's not too far off unfortunately... everyone starts somewhere and that somewhere for me is here, so any help would really be appreciated!

.Net and mono ?

1. Use a cross platform toolkit like Qt, GTK, or wxWidgets
2. Use wine under POSIX and program in Win32
3. Use X11 under both and get an X11 server for windows

I'm guessing that, unless you already know the WIN32 API and are going to leverage that knowledge, you should instead learn a cross-platform windowing thing such as wxWidgets, or GTK+, or some others that I can't think of right now.

Thank you :) Ive decided to learn wxWidget programming because of you guys.
There seems to be so many benifits to it, all except the horribly large executable filesize... Eh but you win some, you loose some.

Looks like I have a lot to learn... like allways