black screen at startup on Vista

Hi,Ok this is getting to be very irritating that I can't figure out whats wrong here. First off after I boot up my computer,it'll say microsoft corporation with the loading bar above that,then it'll show the Vista logo,after that right when the screen is gonna come up to type in my password to enter my desktop,the screen goes black. where I end up having to shut down the computer the improper way.

At times now it does this 2 or 3 times in a row,where then after that everything works just fine as if nothing was wrong. Something weird I've realized after having this happened multiple times. when I start up says the microsoft corporation with the loading bar. Say if that bar loads just for a split second or so then moves on to Vista logo,all of the time it does that black screen after that. where as when the bar loads for several seconds,normally everything works after that. something isn't clicking right I guess.

I've tried re-installing windows,where at first it worked right for 2 or 3 times I started it up and just now it started doing it again where I had to restart 3 times the improper way before it worked. thanks