Open the .inf file with Notepad, stroll down to the line that contains the refreshrate settings for your video card (ie. HKR,, NV25 = GeForce4 ti4600 - if you're not sure what code your card is, stroll all the way down and you'll see it listed), and simply add-in the custom setting you want towards the beginning of the string.
Example: This is what I'm using to set all 16/32bit resolutions from 640x480 to 1152x768 at 100hz for my GF4 under WinXP.
HKR,, NV25_Modes_Delta, %REG_SZ%, "S 640,480,-32,-85-;640,480,-16,-85-;800,600,-32,-85-;800,600,-16,-85-;1024,768,-32,-85-;1024,768,-16,-85-;1152,864,-32,-85-;1152,864,-16,-85-;1600,900,-16,120;1600,900,32,100-;1600,1024,-16,120;1600,1024,32,100-;1600,1200,-16,120;1600,1200,32,100-;1920,1080,-16,120;1920,1080,32,85-;1920,1200,-16,100-;1920,1200,32,72-;1920,1440,-16,85-;1920,1440,32,70-;2048,1536,-16,70-;2048,1536,32,-"
The section I've bolded is the additional string parameter I've entered. You will note that I have "-85-" key for each res. This tells the driver to use the NEXT available res, in my case, 100hz. If you want 75hz, use "-72-". If you want 85hz, use "-75-". etc. etc. etc.
After you're done, save and reinstall with the .inf. Problem solved.
This is EXACTLY the method used by the nVidia Refresh Rate Fix utility, but nVidia changed the .inf format so the older program can't find the strings any more.