Soviet: Do a little google around "Windows 7 Resolution Refreshrate" and it will come up with enough reading material to get you through the next few weeks.
Updating to the latest
drivers is the last thing to do, it will not help, also don't update the videocard
drivers if Windows 7 update offers this.
Why?
Because Windows 7 uses it's own EDID file over the monitors native EDID file. Since this Windows EDID file contains less resolutions/refreshrates, you won't be able to choose those that are not in that EDID file.
In your case, the loaded EDID file will not contain your 1900x1200 res therefore you are not able to choose that resolution. Windows forces you onto 1600x1200 instead as it "thinks" it is the resolution which will work best on your monitor (Windows 7 still has Windows Vista tics here and there).
Workaround:
You probably have to create you own custom resolution (1900x1200). Once this is done, it will show up in the list of resolutions to choose from. How to do this you will find in the various results Google will come up with.
Example:
I still use a IIyama CRT monitor which i like to run at 1154x864 @ 100Hz. This worked fine under XP but Windows 7 simply did not offer this refreshrate. I only got 60Hz (not the least because Windows "thinks" that everybody had an LCD these days). So i made my own custom resolutions @ 75, 85 and 100Hz.
This didn't work at the beginning because i used either the latest nVidia
drivers or if i returned to a bit older drivers, i let Windows update do it's work. Then after quite a search i found out that drivers over 182.50 don't allow you to create custom resolutions anymore. I then installed 182.50 (or 185.52 can't remember) and from then on everything was possible again (but still needed to create those resolutions you want).
When i'm home i will look up the links to the forums i got my info from and post them here (i still have those saved somewhere).
I'm sure when you do a little search here you will find already some stuff.