modify r_mode cvar or remove it from autogen.cfg
itll be in either
c:\documents and settings\your username\local settings\application data\tremulous\base or
c:\documents and settings\your username\application data\tremulous\base
the path might be wrong but meh should help you find the file
another way would be to run trem from a console and add +set r_mode 6(which is 1024x768)
to the devs: would be nice if maybe if the resolution fails it could maybe revert to 640x480(failsafe setting since all systems should atleast support this) that way we don't get questions like this?
EDIT: david beat me to it try his path first
