You can get all the separate "stems" (song elements like bass, guitar, vox, etc) from iTunes for $.99 a pop. Those files will give you an access code for Garageband or whatever music software you use. Once you've mixed, added, and remixed to your heart's content, upload it to www.radioheadremix.com where fans can vote until May 1st.
You have one month to make Thom Yorke proud. Get at it!