I found this posted on the Microsoft forums. Sounds relevant:
Check and see if KB933246 is installed on your computer, if it is remove it.
If you don't want to remove it, or if it is not installed, as was my case, inport the following registry key:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WINEVT\Publishers\{51bd93ee-1739-4261-b8e8-67970423a2a0}]
@="Microsoft-Windows-BitLocker-DrivePreparationTool"
"ResourceFileName"=hex(2):25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,\
00,69,00,6c,00,65,00,73,00,25,00,5c,00,42,00,69,00,74,00,4c,00,6f,00,63,00,\
6b,00,65,00,72,00,5c,00,42,00,64,00,65,00,43,00,66,00,67,00,4c,00,6f,00,67,\
00,2e,00,64,00,6c,00,6c,00,00,00
"MessageFileName"=hex(2):25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,\
69,00,6c,00,65,00,73,00,25,00,5c,00,42,00,69,00,74,00,4c,00,6f,00,63,00,6b,\
00,65,00,72,00,5c,00,42,00,64,00,65,00,43,00,66,00,67,00,4c,00,6f,00,67,00,\
2e,00,64,00,6c,00,6c,00,00,00
"Enabled"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WINEVT\Publishers\{51bd93ee-1739-4261-b8e8-67970423a2a0}\ChannelReferences]
"Count"=dword:00000002
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WINEVT\Publishers\{51bd93ee-1739-4261-b8e8-67970423a2a0}\ChannelReferences\0]
@="Microsoft-Windows-BitLocker-DrivePreparationTool/Admin"
"Id"=dword:00000010
"Flags"=dword:00000000
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WINEVT\Publishers\{51bd93ee-1739-4261-b8e8-67970423a2a0}\ChannelReferences\1]
@="Microsoft-Windows-BitLocker-DrivePreparationTool/Operational"
"Id"=dword:00000011
"Flags"=dword:00000000
After I imported the key I booted with only the Microsoft services running and ran the KB947821.
I installed both SP2 and IE8!