Running Windows XP fully updated. Office 2010 also fully updated. During a regular automatic update, Microsoft Windows Update failed Error Code: 0xB56 for Office 2010 Service Pack 2 for Microsoft Office 2010 (KB2687455). I could not find Error Code 0xb56 anywhere. The error repeated on trying to update again. I had also selected for update a very old Intel Express chipset drive update (which I had chosen to ignore around 2005!). The Intel update installed once then automatically tried again and failed.
I used System Restore to revert to an earlier position but the Office update failed again.
I again System Restore'd, then repaired Office using the Control Panel Add/Remove Programs tool, the repeated the Windows Update (without the Intel chipset update). This time, success!
I suspect the Intel driver update failure was behind this but I also confess to having used Windows Installer Cleanup (from http://www.kztechs.com/wicleanup/ from a lead elsewhere in answers.microsoft.com) to remove some big "junk" files from C:\windows\installer in the middle of all this, so I'm not sure exactly what caused the error. In any case, I found a way out of the apparently undocumented Error Code: 0xB56 so thought it appropriate to share.