There are few external solutions to add IrDA to PC's, including the new
motherboards already come from factory with all the necessary chips
on-board.  Some of those motherboards you just need to plug the IrDA
transceiver (that goes on the PC front panel).

Note that just plug the transceiver is not enough to communicate via
IrDA.  There are three possible ways to physically communicate via IrDA
chips.  Raw mode, ASK and IrDA.  The Raw mode is purelly an infra-red
signal, like the TV remotes, the ASK can use high frequency
communication but still in somekind of raw (with or without checking
characters), while IrDA itself is a new subset of the HDLC, a high level
communication protocol with several software layers and controls. 

Windows (Microsoft) offers a software package to use IrDA at your PC
under Win95/98, but it uses the regular IrDA procotol for printers and
notebooks (only).

For a simple communication between a PC and a PIC, you can use the raw
or ASK mode that would be plenty.  Remember that IrDA recomendations
produces devices that can only communicate in a cone of aprox 20 to 30¡,
in a maximum of 1.5-3 meters distance.  Remember that you would need to
write the software for both sides, PIC and PC.

For IrDA chips (transceivers and UARTs) take a look at HP and Cirrus
(Crystal) company. Temic (Telefunken) also produce those devices
(TFDS3000/4000 and TOIM3232) but they have a BIG delivery problem for
quantities lower than a full reel.