Just in general, which OS are you using?
For your problem:
Your computer cannot display this resolution.
You either edit glest.ini ( search for 1024 ) and setup your native screenreolution there. Or you use the glests configurator tool which is a separtare program that comes with glest and change the the settings there.
Just because I'm curious:
What kind of computer cannot 1024x768? Some sort of netbook?