When you dock your Zune to update it, if certain software updates are not available, the update may fail.
This is documented in this KB article:
KB944938:
A required component cannot be installed when you try to install the Zune software
http://support.microsoft.com/default.aspx/kb/944938
The solution is to basically download the Zune Client software (2.1) seperately, extract it, and run the Startzune.exe file from the unpacked directory to complete the installation.
Zune Client software, version 2.1 (166MB)
http://www.microsoft.com/downloads/details.aspx?FamilyId=6136349F-2B32-4946-83B5-A09775531EF4&displaylang=en
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