|It could have been done in Display Properties > Appearance > Advanced but it would be far too detailed for you to fathom and I am not aware of anything there being set as low as 20.|
Increasing the DPI via Display Properties also looks far too detailed, involving a drop down. I'm tempted to think this was what was attacked (dropped from 100% to 20%).
For what its worth in Safe Mode it should be back to the size before you accidentally moved the slider in Display Properties.
I'm now like you, stuck. I will keep thinking about it and if suddenly inspired I'll shout back. Just one thing, when you get the Safe Mode menu is that normal size? Not that I have any brilliant idea but it might tell me something.
The only other options are reloading Windows (or Dell restore if you have such thing) but I assume you don't want to go that way.
One more question. When you tried XP Repair is this the procedure you used?:
If so were the blue screens too small as well?