screen resolution!

Hi people, This morning i was told by a freind to install a game called diablo 2, again. Last time i played this game everything was fine, no problems at all. This time, i actually cannot run the game. The problem: my moniter cannot run 800x600 or 960x600. But anything higher it can. The game, diablo 2, is run in 800x600. Also, when i try and go to Display in control panel and try to change the resolution to 800x600 or 960x600 my screen goes fully black and the light (indicating the moniter is on) flashes fast. and stays that way. why does this happen now and not back when i used to play it? Thank you in advance. Lochy.

ok, i went to control panel>display>(tab for my vid card). then went to screen reso on that tab.. its got all the reso's that i can run on this moniter there, (theres a little button underneath the resolution bar saying "hide resolutions that this moniter cant run" i clicked this and it took away alot of the notches for resolutions) but the resolution that the game is (800x600) is there, saying i CAN run it. i cant run 960x600 either.. theres alot of resolutions between 800x600 and 960x600 and i can run all of them fine.. but its just those 2 i have troubles with!! how annoying any help would be greatly appreciated. Thanks! edit! i managed to get 800x600 working.. but only the title screen of the game is like that and the title screen has no options.. i think the gameplay is 960x600.. i changed the hertz to make the 800x600 work.. but all the hertz changed u can do for 960x600 all dont work.. ty