Try reinstalling GTK.
I tried that already. No luck.
And again we hit the problem that 99% don't even know the name of the program you use.
GTK = Gimp Tool Kit.
Radiant = Map editor.
GTK Radiant = Version of map editor that uses GTK to draw the windows.
As it has GTK in the name I'm assuming it used to use a different tool kit, and was then ported over.
It looks to be a font issue, so try reinstalling GTK and/or change the font.
Does Radiant have any internationalisation stuff? As only some text is fucked it could be that the language files are corrupted, although a reinstall should have fixed that.