There are modules (receiver and transmiter) from AUREL working at about 430MHz and are suitable for about 50m. If you need bigger distance, i can search for you but let me know.
At the side of PC you will need to convert RS232 signal to TTL signal before applying it to this module and vice versa. To do this, try an IC like Max232 from Maxim or something similar
there exist single-chip solutions that do exactly what you want: you can
feed them a RS-232 signal, they send it over a radio-link (433MHz,
868MHz or 2.4GHz) and can also receive and demodulate such a signal.
Such a chip is e.g. TR1001 from RFM (look in google). Also nordic has
such chips.