RC5 has something known as a toggle bit. That is there is a bit (1 binary digit) inside the code that changes from 1 to 0 on each transmission. Other software only solutions can send RC5. Not sure about others. You might want to check out the JP1 forum on Yahoo.
Note that you can download ProntoEdit 4 from this site (Classic Pronto Files Section), install it on XP and give it a whirl. You can also have a look at the Pronto FAQ where toggle bit codes are discussed.