Windows wont start and now wont let me reinstall

hi for a while iwas noticing that my computer was slowing down (q9450 evga 780i evga gtx280 4 gb ram 4 hard drives 2 250s 1 500 1 raptor 36gb) soo i thought i would restart to see if that would help then after the green loading bar goes the screen just went black and did nothing for a reallly long time soo i reinstalled windows and it worked fine i installed the updates the updates needed to restart and it did the same thing got stuck at the black screen i did notice that the install took longer then normal but once i got it working it was fine
any help would be very nice