How can I convert any .htc file into .js and use the resulting JavaScript?
For example, this is a .htc file: http://curved-corner.googlecode.com/files/border-radius.
It is not possible to convert any .htc file to an "equivalent" .js file. That is, it's not possible automatically, in general. However for the specific file you've posted, you could convert it using jQuery's live event.
But anyway, there's plenty of Javascript-only libraries that do the "border-radius" thing, why do you need to specific convert this one? A quick google search turns up dozens.