That's because the Media Center Edition is an extension of XP Professional. There's a few little pieces added in that make Pro into MCE. Not a great deal of difference, and even if you did end up with Pro instead of MCE I doubt that you'd notice the difference.
On an interesting side note, I have Avast! for Linux working on my Puppy test box! I made a self-installing .PET file of it, and if it proves beneficial, I could easily have it running on yours too. I think F-prot has done the job, but if we need something Avastier to finish the job, now we can.
I frankly wish I had tried this sooner, as it was so easy to get it working that we could have used Avast! for Linux to scan instead. Next time.......