Tried to update our oldest SQL 2005 box to SP2 this morning, and the update of the database and analysis services failed. Nothing too obvious in the log. During the middle of the SP install, I got this error twice:

Installation of KB921896 Failed.

I'll spare everybody the hour or so of blog reading; here's what I ultimately found:

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=378205&SiteID=1

This server was originally just a member server, but I upgraded it to a Domain Controller (long time ago, back when I had very limited server resources). The security groups that the post at microsoft.com refers to were deleted at DC Promotion. Removing the registry entries mentioned in the post worked great.