While you are in DOS:
Navagate your way to your Windows Folder, and type:
setup.exe
Once that loads up (you may need your original disks) change your monitor settings. Just mess around with them until you get a combination of settings that works. (SVGA works well on my older computers, SuperVGA, and as for resolution, its Windows 3.1, should it really be 800X600? Try another one, and see if it improves. )