Error Installing 2003 Server Service Pack 2 - Access Denied

When installing Service Pack 2 of Server 2003 I have seen twice a "Access is Denied" error appear... with no additional information. One was on an SBS server and the other was a Server 2003 standard. Both of these problems were caused by exactly the same thing. The setup tries to delete a registry key as below. The Administrators group only had read access and not full control - I simply used regedit to assign full control to HKLM\Software\Microsoft\Windows NT\CurrentVersion\SvcHost

Below is an extract taken out of c:\windows\updspapi.log

#W190 File "C:\WINDOWS\system32\SET16F5.tmp" marked to be moved to "C:\WINDOWS\system32\dpcdll.dll" on next reboot.
#-086 Deleting registry value "HKLM\Software\Microsoft\Windows NT\CurrentVersion\SvcHost\wugroup"
#E033 Error 5: Access is denied.
#E065 Parsing DelReg section [Product.Del.Reg] in "c:\03999bc3e44d3be7bfb12644\i386\update\update.inf" failed. Error 5: Access is denied.
#E064 Parsing install section [ProductInstall.GlobalRegistryChanges.Install] in "c:\03999bc3e44d3be7bfb12644\i386\update\update.inf" failed. Error 5: Access is denied.
#-086 Deleting registry value "HKLM\Software\Microsoft\Windows NT\CurrentVersion\SvcHost\wugroup"
#E033 Error 5: Access is denied.
#E065 Parsing DelReg section [Product.Del.Reg] in "c:\03999bc3e44d3be7bfb12644\i386\update\update.inf" failed. Error 5: Access is denied.
#E064 Parsing install section [ProductInstall.GlobalRegistryChanges.Install] in "c:\03999bc3e44d3be7bfb12644\i386\update\update.inf" failed. Error 5: Access is denied.