Find and edit the following config file with notepad:
C:\Program Files\Call of Duty\uo\uoconfig_mp.cfg
For W7 and newer it is in:
C:\Users\yourusername\AppData\Local\VirtualStore\Program Files (x86)\Call of Duty\uo\uoconfig_mp.cfg
All of the following commands need to be changed in the file (they are already there but with different values). Obviously only one of 16:9 or 16:10 needs to be done, depending on your screen. Replace 1440 and 900 with your monitor resolution (1920 and 1200 for me).
For a 16:9 Aspect Ratio Resolution: (usually for widescreen TV)
seta cg_fov 96.4183 seta r_mode -1 seta r_customheight 900 seta r_customwidth 1440
For a 16:10 Aspect Ratio Resolution: (usually for widescreen pc monitor)
seta cg_fov 90.3951 seta r_mode -1 seta r_customheight 900 seta r_customwidth 1440
Once done, when you go in the game menu, it may not show the correct resolution in the options, but it will be in game. Do not change your resolution in the game. If you do, you will lose widescreen, and have to do this all over again.