In the launcher where I select my resolution, all of the options except the maximum resolution are displayed with a letterbox. Is there any way stretch a lower resolution to fill the screen so that I can play without the letterbox?

Usually, in my experience, this would be a function of your video card drivers/software and/or monitor functions.
However if you 'zoom' or stretch to fill the frame images may be either distorted or cut-off. Depends on the reason for why it's letterboxing.

Right, but it certainly wouldn't hurt to make a more complete version of a problem report, including driver IDs and dates.

Processor Manufacturer: AMD / Intel
Processor Name / Type: Athlon X2 / Pentium D / C2D / dual core / quad, etc.
Processor Speed: ?.? Ghz
Operating System / Service Pack: WindowsXP with SP #?, Win7, Vista ?
System RAM: ? ? GBs (MBs, if less than 1024 ? )
Video (GPU) Manufacturer: ATI / Nvidia (only those two are supported, PERIOD, so please answer which of them you have)
Video Card Model: (examples) Radeon HD 5670 / Geforce GT 240
Video Card Driver Version: Catalyst ?.? / Nforce ??.??
. . (please use the numbers, not the word "latest")
Video Card onboard RAM: ? ? ? MBs
Sound Card Driver numbers and dates
Sound Card Manufacturer:
Sound Card Model: