There's a really annoying bug in ET that happens for some people. Say you set your resolution to 1600x1200 in the options menu. Then you go and play on a shrub or etpro server. Then, you exit the game. Then, you run the game again. When the game loads up, it will be reset to 800x600 resolution!
If you are a victim of this bug, just add these three lines into your autoexec.cfg:
set r_mode -1
set r_customwidth 1600
set r_customheight 1200