Just removed DRM from some 10+ free AZW ebooks from Kindle Store using Kindle for PC 1.4.0, Calibre 0.8.1, K4PC/K4Mac/Kindle Mobi and Topaz DeDRM 0.2.8. Works like a charm.

Have you downgraded your K4PC? Then delete the contents of the My Kindle Content folder and then re-download your ebooks.