its a rotation setting deffinatly trust me on this. it would be correct all the way up to the loading of the video driver because the monitor display's in VGA mode when booting up so that it is compatible with all forms of video cards.
try this:
Right Mouse Click your Desktop>>Properties>>Settings>>Advanced>>Adapter
if it is Nvidia go to
http://www.nvidia.com and get latest drivers and do this step.
Right Mouse Click your Desktop>>Properties>>Settings>>Advanced>>""there should now be a tab with the name of your video card on it"" click on it>>look for a tab along the side called NV rotateand make sure it is set to Landscape 0, if it is. change to portrait, then try inverted landscape, then try inverted portrait untill it comes up right.
if your adapter says ATI or Radeon. go to
http://www.ati.com and get the drivers and do this.
Right Mouse Click your Desktop>>Properties>>Settings>>Advanced>>""look for a tab with the name of your video card"" click it>>steps should be similar to the nvidia card, i dont personally own any ATI cards, but the menu on the side i believe is the same as the nVidia one.
happy searchin'.