The 433MHz board with radiohead library works great for communication between two arduinos but is there a way I could transmit from a PC to an arduino over 433MHz? Just thought I'd ask before I do something like tether another arduino to my pc to act as a 433 MHz UART repeater - perhaps there's a more straightforward way. Thanks!
No. An MCU/transmitter using the RadioHead library is required to talk to an MCU/receiver using the RadioHead library.
There are "wireless serial" adapters that allow wireless communications between a PC and an Arduino, but you need two of the same type.