The AAC format is more formally known as MPEG-4 Part 3: Audio, thus m4a. MP3 is named so because it's MPEG-1 Audio Layer 3.
You can play MPEG-4 video (usually mp4) and audio (AAC) files in Windows Media Player if you install the MPEG-4 codecs. Microsoft isn't too interested in directly supporting such ISO standard video and audio formats that compete with their own formats. I believe FFDShow is what you're looking for.
Also, do you know if every song is available without DRM for $1.29 or just a selection?
Only for those labels that have allowed Apple to make them available that way. And the songs are 99 cents now just like the rest.
I may be tempted to make a temporary iTunes account and use this card myself.