It turns out that once you set a font for document classes, you can't set the font for a document class back to the default (on Windows XP).
If you edit the registry, and rename or remove all the font keys under
HKEY_CURRENT_USER\Software\Helios\Textpad 4\Document Classes\[class here]
then textpad will display documents with the default font. You have to close and reopen Textpad for this to take effect.
Of course, as always, do not edit your registry at all. If you do, back up the registry before editing. Editing the registry can make your machine unusable.
Looked forever, couldn't find anything documenting this, so posting it for other frustrated users.
Set Textpad font back to default
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