Have you tried downloading the newest driver for the monitor? YOu didn't post which model it is, but if you do, I can post a link that leads directly to it, if a new driver is available.
I just put a new computer together and fired everything up, installed windows vista ultimate and everthing has been working for about seven days with no problems. I purchased a new monitor (Samsung 22" wide screen) and hooked it up and it has been running great until now. It boots up and then it shuts off. I hooked up the old LCD widescreen(LG) and it works fine. Hook up the Samsung and it boots and then shuts off. Could it be a faulty monitor? Any ideas before I take it bake? I also tried an old monitor VGA and it also worked fine.