Generally, you just pull up the Vista control panel from the Start menu. Where and how depends on whether you use the Visa themes or the Classic theme, and a couple other factors - just look for the Contol Panel or Settings icon. You can also just type
control
into the Run box and hit enter.
If you have the category view, the graphics properties would be under either System and Maintenance or Appearance and Personalization. Just look for the graphics properties under one of these. As I said before, what the icon's name is will depend on your graphics chipset.
Or maybe you'd rather do it this way, because you won't have to look for it:
Right-click any empty space on the Desktop and select Properties → Settings tab → Advanced → then select the tab with the graphics chipset vendor's name on it. For example, "Intel Graphics Media Accelerator Driver".
Yet another method: if there is a graphics icon in the system tray/ notification area nect to the clock, simply click on that. You can try right-click to bring up a menu, or left-click to open a dialog box.
Further: You may be able to use a key combination like
ctrl + down arrow
alt + down arrow
ctrl + alt + down arrow
These may or may not work depending on how the hotkeys are set, but there is likely some key combo which does this - that is the most common cause for having a surprise screen-reorientation.
From this point, it depends on how the graphics vendor set up their control panel applet, but you will likely have the choice of pushing one button which brings it up. You then want to look for something like "Display Properties" - don't be afraid to click tabs and menus until you find it, if it is not obvious.
Eventually, you will probably find a section mentioning Rotation, and you'll likely see a little picture of a screen with a radio button on each of its sides. Click the radio button at the bottom of the picture (the one opposite the button which is currently marked).
Again, after this, I would suggest looking in this control panel applet to find the hot keys properties and turn it off, or turn off the ability to change screen rotation entirely. You don't want to go a couple days with an upside-down screen again, right?
I hope that is descriptive enough to help. I'm sorry I can't be more specific with the instructions, but the graphics properties setup varies a bit between vendors and the the different drivers provided by thoise vendors.
Sorry if this is long, I'm just trying to offer as many options as possible.
Good luck! Post back if you still need clarification. Be specific as to where you are getting stuck and we'll try to help. :)