Really, we need more information :]
OK, have you installed the drivers?
How long have you had it? New?
What make, size?
What resolution is it set to?
I presume you're using Windows Vista.
What GPU are you using? Maybe give us some specifications on your entire system is nice. :]
What cable are you using?
What refresh rate is it set to?
Sorrii to throw so many questions. Answer what ou can and that'll be useful. :]