I have no clue what the actual program was because I did not participate it. As far as I understood it, it was the situation where one purchased a computer and the default OS was Vista. The person talks to either the vendor who gives them the right number to call, or calls MS directly. The program essentially exchanged Vista for XP. There may have been a price to pay to get the XP discs sent to you, or you may have gotten a refund, again I have no clue. However since the OS would be an OEM version, the replacement program may have been at the retailler’s level in place of an actual Microsoft incentive.
However, there does seem to be an option to “downgrade” one’s Windows Vista Business and Windows Vista Ultimate OEM license. Here is the link to the cached PDF page where this comes from. However again, this may be expired, and it seems it only works with certain licenses of Vista.
Your best bet is to contact the company the computer was bought from and see what they say, then ask MS too.