1. Remove iTunes DRM protection using iTunes + CD-R

Follow these 5 steps:

1. Create an iTunes playlist containing your purchased songs.
2. Insert a blank CD-R or a CD-RW.
3. Burn the playlist to disk.
4. Re-import the songs after a successful burn.
5. Deleting the original purchased tracks.

2. Remove iTunes DRM with DRM software in Windows

One of these DRM Remover is Daninusoft Media Converter Pro which can remove iTunes DRM with its newest version (2.2.4.0). The reason I list it here is the newest version has modified its two major disadvantages compare with the former one:
1. Be compatible with Vista OS. 
2. Supports all iTunes DRM files. And with its other advantages: High conversion speed and friendly-use interface it could be a good choice to remove all DRM files.
Free download: