I've been looking into this and have found that there are several options for both the device and the software. First word of caution, DO NOT BUY OR ATTEMPT TO USE an IrDA device. Chances are high that you will not be able to make it work.
Here is a page on the supported devices of uICE - a freeware software product for controlling the PC using an IR device.
Also, check out
www.ir2pc.com.
Finally, it appears that the unit featured here could be a good option - it is pretty cheap, has the IR device and the software, and already has codes assigned for learning, so setup would be pretty quick.
Good luck
-Curt