With ClearType turned off there is really no hope. Even native system fonts look bad with ClearType off.
I believe IE8 and above automatically override the OS settings on XP and turn ClearType on, but I don't think there's a way for a non IE browser to do that.
If you care about the relatively small percentage of visitors who are in this category you'll need to spend many hours choosing and testing a better font.
My advice is to not worry about it - Those users will be quite accustomed to seeing horrible fonts all over the web because of their ignorance of or inability to change system settings.