There's several things that you can do here. The first is to see what Vesa will do for you (when you see the Slax boot screen from a flash drive or CD, choose "Slax Graphics Vesa Mode". Now, if that doesn't work, then I would try pressing Tab and then would see what vga=791 would do appended to both the normal Graphics mode and the Vesa mode.
Now, if the drivers for X just can't support your monitor (i.e. those tips don't solve the problem), which is the case most of the time these days, there's several routes you could go:
- Take a look at the Porteus project at porteus.org and give it a spin.
- Download + add modules for your display adapter from here.
- Boot with "Slax text mode", then try to run xconf before startx.
- Manually compile a new copy of the X window system & kernel modules.
I hope one of these methods works for you; good luck! :)