Sadly, the Archlord UI does not yet support widescreen resolutions. It is possible to manually change the resolution to a widescreen resolution, however only the game area will be rendered in widescreen. The UI (bottom bar with skill slots, health, etc), will still be rendered in 1024*768, and thus probably be positioned in the center of the screen (which is rather annoying).
If you can live with this, the resolution can be changed by manually editing "COption.ini" in the "ini" folder within your main Archlord folder. Simply change the "Height" and "Width" parameters to your preferred widescreen resolution, and save the file.

If you play at a 4:3 resolution on a widescreen monitor, you can use the ingame command "/wide" (simply press 'enter' and type "/wide", followed by 'enter' again), to change the aspect ratio to better fit the widescreen ratio. This will make the game appear less stretched than usual, although it will still run at the original 4:3 resolution.