Monitor is dark

I am having trouble with the monitors on both of my computers. They are terribly dark, so that you cant see some/most pictures without brightening them with software. I hooked an old Compaq Presario 140 monitor up to this computer and it was perfectly bright. But the other monitors (one an HP 15″ and the other a Daytek 17″) are both too dark. I have the settings on the monitors all the way up but it is still way too dark. What do you think the problem is, and is there anyway to brighten everything through windows? Also I tried to play a game and it was too dark to make most things out. Any help would be appreciated.