Answer:
On your monitor are several little buttons. Push these buttons to calibrate the display on your screen. There are options to bow the sides of the display in or out, make the display taller or wider, or to even tilt the display (which is what I'm assuming you mean by sideways). Once you find the option in the menu with the buttons on the monitor to tilt the display, use the other buttons to "move" the display about on the screen until you've gotten it straight.
Or just turn your monitor right-side-up if you've got it sitting on its side.
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Okay, I'm guessing you pushed CTRL + ALT + Delete to pull up the taskbar but instead made the screen go sideways. If you want to fix that just push CTRL + ALT + Up arrow. And if you want to make it sideways again push Ctrl+ Alt + Right arrow.
Note: the CTRL + ALT + Arrow key fix only works if Hot Keys is enabled, and not all XP installations have it enabled, so it won't work for everyone.