ClearType is Microsoft's marketing term for their implementation of SubPixelAntiAliasing.
Although this isn't Linux-oriented information, you can activate ClearType support on your copy of Windows XP by going to:
Start menu -> Control Panel -> Display
Click on the "Apperance" tab, and click on "Effects..."
Check the box next to "Use the following method to smooth the edges of screen fonts" and use the drop-down menu to select "ClearType"
One page links to ClearType:
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