MS Update - Error Code: 0x643

Any ideas please? I have an HP AMD x64 dual proc. running XP Prof. x64. I

do not use automatic updates so I have just run the updater but one update,

Security Update for Microsoft XML Core Services 6.0 and Microsoft XML Core

Services 6.0 Service Pack 1 (KB933579), keeps failing with the following

error: Error Code: 0x643

I have tried the update manually from the MS download centre but it still

will not update.

Regards,

David