You've not mentioned what OS you're using.
Assuming you've got an LCD display, the pixels are vertically divided into three subpixels of red, green and blue. The Antialiasing performed by recent OS's is clever enough to know this and take advantage of the fact that lighting up a pixel red actually only lights one third of the pixel. Therefore the OS can draw fonts with subtly coloured edges that fools the brain into thinking the font looks smoother.
When you turn your monitor by 90 degrees, the pixel is now divided up horizontally. If you leave the font settings as they are, the OS will still be working on the assumption that the pixels are vertically divided, and so everything will look rather wrong.
Have a look around in your system or graphics card settings to see if there's a setting to allow sub-pixel antialiasing for horizontally divided pixels.