DVi LCD Problems w/ Samsung 226bw

I bought a Samsung 226bw and wanted to use it in dvi instead of analog since its better.
I tried it on my laptop with my gf 7600 GO and it didn't work
tried it with my 7800gt and it didnt work either
then i tried it on my gf 4400ti ... and it didnt work in dvi either but it worked on analog..............
i know the monitor works properly since i can use it with analog but i bought it for DVI and HD... do i have to do something special?? this is my first time with lcd monitors too.. always used CRT
i tried to extend the desktop to a second monitor with windows xp but the monitors stays black or kindo messes up and become white with weird colors
when i plug the monitor in my computers, the computers detect it as samsung 226bw but the DVI image wont come out on the monitor
Hhhheleellllllllllppppppppp!!! I bought this monitor on ebay and it has no warranty... plz dont tell me its screwed up!