How do I manually change the DPI settings?

how do I change windows DPI (windows 7 or XP) to ';175%'; instead of the only option of 150% or 200%How do I manually change the DPI settings?
On the left hand side of where you see the options for smaller, medium, and larger you'll see a link for ';Set custom text size (DPI)'; click on that and type 175 into the box.