Find your xorg.conf file and scroll down to the screen section and see if you find something like this:
Viewport 0 0
Modes "1280x1024" "1152x768" "800x600"
I don't know what you have for a video card but it could be that a mode, i.e., resolution is specified that the card can't handle. It should select a resolution that it can handle from the list but you never know. If the Modes line isn't present you might add it with a basic resolution, like 800x600, that you know the card can handle. You can experiment with various resolutions in the other color depth subsections until you find something that works. You might start with something like 800x600 in color depth 8, then work your way up through the various resolutions in each color depth until you find the limits of the card.
That said, if you have a pretty good video card it should easily handle something like 1280x1024 at a 24 bit color depth. Googling for your card's capabilities should prove helpful and will possibly suggest a starting point.
Finally, Slackware has a utility called xorgsetup that will probe your card (and other hardware) and setup the xorg.conf file for you. Perhaps Mepis has a similar utility.
Above all, don't worry, this is a solvable problem.