ClearType is disabled in Windows XP, and enabled in Windows Vista, 7, 8 and 8.1 by default. On different monitors, this has different effect on how clear or readable text on screen is. Generally, ClearType is more useful on LCD monitors.
In Windows XP and Vista, you can turn font smoothing and ClearType on and off; in Windows 7, 8 and 8.1, you can also adjust font smoothing to your own requirements.
Here is how Windows XP looks on my LCD monitor with Standard font smoothing, ClearType font smoothing and no font smoothing:
Here are the same examples for Window Vista:
And here's Windows 7 with ClearType enabled and disabled:
In Windows XP, right-click an empty area of Desktop and click Properties.
In Windows Vista, right-click an empty area of Desktop and click Personalize.
In Windows 7, open Start menu by clicking Start button or using keyboard shortcut Ctrl+Esc. Type "cleartype" into Start menu search box and click Adjust ClearType text.
In Windows 8 and 8.1, use keyboard shortcut Windows Key+W to open Settings search, type "cleartype" into search box and click Adjust ClearType text.
Touch screen users should swipe in from the right edge of screen and tap Settings first.
In Windows XP, Display Properties window opens. Click to open the Appearance tab and then click the Effects... button.
In Windows Vista, Personalize appearance and sounds window opens. Click the Window Color and Appearance link. This opens Appearance Settings window where you should click the Effects... button.
Select ClearType or Standard from the Use the following method to smooth edges of screen fonts box. To turn text smoothing completely off, clear the check box. Click OK in the Effects window and click Apply in the previous window to see the difference.
That's it for XP and Vista.
In Windows 7, 8 and 8.1, clear or check the Turn on ClearType check box in the ClearType Text Tuner window. The difference will be visible on screen immediately. Whether you decide to use ClearType or not, you will have to go through four (Windows 7) or five (Windows 8 and 8.1) additional steps in font smoothing adjustment for the changes to take effect, so click Next. Each step offers several text samples to choose from - select the one that looks best to you.
After you've made your choices, a summary screen will be displayed. Click Finish.
And that's it for Windows 7 and 8/8.1.