Hello,
These days, most LCD monitors come with a "device driver" CD that contains the profiles for the monitor, which is just a file that lets the computer specific identify the model of the monitor and the resolutions it supports.
If you install the profile from the CD, you should then be able to go into the display settings for your version of Microsoft Windows and select the native resolution used by the monitor.
If, for some reason, your monitor did not come with a CD, you should be able to download the file from support Dell's web site by searching on the model's specific name. Typically, they are very small (under a megabyte).
Regards,
Aryeh Goretsky