HSerIn doesn't need to be used in an interrupt.
If you've got going what I think you've got going...
When your interrupt trips, you've got a character in the receive register. When you get into your interrupt routine, the HSerIn line will wait for the next character, not get the one that's already in the receive register.

And besides that, you're ON INTERRUPT doesn't get enabled until the end of the program (3rd line from the end of your code), which it will never reach.