you would have to practically rewrite the program, I think, to make it a Win32 app. The PicEdit source uses some assembly language, if I'm not mistaken, and accesses the hardware directly. Windows doesn't let you access the hardware directly.
I'm not sure editing PicEdit's source is the best place for you to start learning C (it uses pointers heavily, for starters). Though if you want to try it, go for it. As for writing a Windows program, a compiler that runs in Windows won't cut it. You'll need to learn either the Windows API or a library such as MFC or whatever the Borland library is called that hides the API from you.
I don't mean to be negative here, but how much programming experience do you have? Are you just learning your first "real" programming language (i.e. C or Pascal instead of AGI)? If so, I should warn you that you won't learn the Windows API or any of the Windows libraries overnight.
I also think the most useful extension to PicEdit would be to make it a Windows program, but before you try it, you'll first need to learn C; then once you have a pretty good grasp of C (you should be comfortable with pointers and bitwise operations at the very least) learn a Windows programming library.