I went back tonight to the Circuit City where I purchased the HP system. I went to Customer Service first, who told me to go talk to the manager of the computer department. He told me it was a misprint in the ad and unfortunately there was nothing they could do, although he did say I could return it. He also said that they had posted a sign about the misprint. Maybe they did later in the day, but there was no sign at 5am when I was there, the computer was not on display, and no employee there mentioned this fact to me. Anyhow, I told him this was unacceptable - that their ad constituted an offer, which I accepted by purchasing it. I explained that this was no different than if the ad had said the system had 1gb of RAM and it actually had 512mb.
Anyhow, I then went over his head and asked for the general manager and phone numbers for the district, region, and corporate. I also asked for their store number and started writing down names of employees I talked to. After waiting for the general manager for about 10 minutes, the computer manager and the general manager walked up together and offered to buy an OEM copy of the Windows XP Media Center Edition 2005 online for me and will call me once it arrives later this week. I don't really know how they're allowed to do this, but I think it's an acceptable outcome.
That being said, I'm certainly going to make sure it's a licensed copy and not some pirated burned-at-home copy. Hope this helps.