If you mean you have built this into an executable, and don't have the source code now to edit it, you really do have a problem. There is no way to change anything unless you have the source code.
If you do have the source code, then your only choice to get it to look good on an 800x600 screen is to go back and edit it. There's no magic setting that will fix the fonts.
Fonts are always a problem, especially on Windows. I've seen the same font look different on a PC that I dual boot between Win98 and XP set to the same resolution. It just takes a lot of planning and trial and error to get it right.
Ed