How to enable ClearType font in XP
 

By default XP doesn't have ClearType enabled.

To enable ClearType.

1. Right click on the desktop, select Properties, Appearance, Effects, select ClearType from the second drop-down menu and enable the selection.

Yep, this great tip mentions the easiest way. However, that options that this way provides might not be the best ones suitalbe for you. So here are other things you can do to get clear type.
1. Get the ClearType tuner PowerToy from Microsoft. This will help you to tweak your cleartype settings.
2. Do it online at Microsoft's website, this will also help you tune ClearType to your preferences.

Hm... sometimes I wonder why XP didn't come with ClearType enabled already, like OS X or Linux. Oh well atleast it's good to know that you can turn it on.