Just to add a bit to era's excellent distillation of how X11 font work (which I've now bookmarked for when I try and explain it to people thanks!) - the version of xming I use has a
really small set of fonts by default, your's might well have the same problem.
There's good news though, you can add additional fonts to xming (like, say, courier-medium and helvetica-medium
)
If you already have those fonts available in windows, you can either tell xming where to find these by editing the font-dirs file.
If they are not on your machine at all, you can either install them into windows, or into xming itself.
There's loads of information on xming fonts here:
Xming - PC X Server - Fonts