I don’t know how to call it: a need a program (some kind of a virtual monitor) that will emulate 4:3 screen on a 16:9 monitor (put black bars on left & right side of the screen). Yes, I can set that in screen options but it doesn’t work for one specific old game I want to play, no way (looked through all possible forums). Do you know for any application like that (I’ve googled it but couldn’t find anything like it)? Thank you! 
This may be of some use to you. http://www.developerfusion.com/forum/thread/58182/ This sounds more like a graphics card driver issue more then anything else.
You have to set this in graphic card drivers.
Thank you both, but this is not the solution I’m searching for. If anyone knows for a little program that can change this quickly to 4:3 and back, without messing with the drivers, please tell me.
I have remapped a 1920x1200 to 1600x1200 if this is of any use.
Barco made a 1600x1200 28" CRT Monitor (25" viewable). I designed an external box that remapped the input to the center with two black bars on each side. I used a Dell 2709W monitor that gave me the 25" diagonal.
I hope this helps