The challenge:
Provide dynamic web fonts that render acceptably across all major browsers, devices and Operating Systems
The story:
In my case, I found that the answer does not in fact center on enabling Clear Type in WinXP. Whether Clear Type is enabled or disabled, I find that some of my @font-face English fonts (especially BLACK, SANS-SERIF) look bad in IE7 and IE8. It's sometimes more than just being jagged or having bad anti-aliasing. The big problem I see is that the shape of the font itself (some fonts, not all) seems to squish into something quite unattractive. I found an acceptable solution by following the advice of Torben. Font Hinting was all I needed, and again, that solution has nothing to do with Clear Type being enabled. To get the font hinting app up and running, read the instructions here:
https://gist.github.com/davelab6/3783491
(I used the "ttfautohint-0.95.tar.gz" file here): http://sourceforge.net/projects/freetype/files/ttfautohint/0.95/
You should also review the documentation, and you need to also install MacPorts (or you will get an error in the Terminal during your build), but since my rep score prevents me from posting more than two URLs, you'll have to Google for that.
Better hinting doesn't mean your @font-face fonts will now look as perfectly smooth and great on IE in WinXP as they do on Macintosh browsers. But I found that the font hinting prevents nasty distortions in the shape of some fonts within IE7 and IE8. To me, that is an acceptable solution.
I can only add that it's rather troublesome to build the ttfautohintGUI on OSX. You have to installed hundreds of MB of software, downloading from multiple locations too, just to build the GUI app. But once done, the app allows you easily to hint your @Font-Face fonts and resolve many WinXP display issues. Just be sure to tick "Windows Compatibility" in the GUI (it's unticked by default). I also unticked the "Add ttfautohintinfo" and "Pre-hinting" options, leaving everything else as their defaults. Click "Browse..." to add your Input File (the font you want to hint), and then copy and paste that path in the Output File, changing the filename to whatever you want it to be. You won't enable the "Run" button unless there is something in both the Input File and Output File fields.
Best wishes.