Ze względu na charakterystykę problemu opiszę rozwiązanie w wersji angielskiej :).
Well the solution is very simple most common cauuse of the SXS_ASSEMBLY_MISSING error is that from unknown for me cause service pack is looking for some language MUI which the system dont have. This is especially typicall for laptop users. Looks like that this is an error on service pack.
The place were you should look first is thee installation log located at c:\Windows\Logs\CBS\CBS.log
Then You should look for the first line with ERROR on the left
For example one of Windows forum users have this :
2011-02-25 20:31:49, Info CBS Exec: Resolving Package: Microsoft-Windows-Client-Drivers-Package~31bf3856ad364e35~x86~pt-BR~6.1.7601.17514, Update: INF_netrndis
2011-02-25 20:31:49, Info CBS Exec: Resolving Package: Microsoft-Windows-Client-Drivers-Package~31bf3856ad364e35~x86~pt-BR~6.1.7601.17514, Update: INF_netrndis, PinDeployment: x86_netrndis.inf-languagepack_31bf3856ad364e35_6.1.7600.16385_pt-br_5a74a56194bd58af
2011-02-25 20:31:49, Error CSI 000001a3@2011/2/25:23:31:49.483 (F) d:\w7rtm\base\wcp\componentstore\csd_locking.cpp(324): Error STATUS_SXS_ASSEMBLY_MISSING originated in function CCSDirectTransaction::LockComponent expression: (null)
It is easy to assume that he does not have pt-BR MUI which for some reasons Windows wants.
The easy step to help yourself out is to install that language pack just for the time of Service Pack installation.
Go to http://www.froggie.sk/ install vistalizator, then install the language pack you need and you’re …………… not done yet 😀
The problem is that there are updates which need to have all mui installed and you have them already, this is an easy step because you will get an error during Service Pack installation.
Open CBS.log mentioned above and again look for the first error line for example :
2011-03-02 19:08:13, Info CBS Exec: Resolving Package: Package_4_for_KB983590~31bf3856ad364e35~amd64~~188.8.131.52, Update: 983590-27_neutral_LDR, PinDeployment: amd64_a3a447e7a026fd8b08eef9015c3fd303_b77a5c561934e089_6.1.7600.20717_none_985a0e1bc62dd1b4
2011-03-02 19:08:13, Error CSI 000001f1@2011/3/2:18:08:13.668 (F) d:\w7rtm\base\wcp\componentstore\csd_locking.cpp(324): Error STATUS_SXS_ASSEMBLY_MISSING originated in function CCSDirectTransaction::LockComponent expression: (null)
Above you see that there is a problem with KB983590 just uninstall it and start all over again, there may be more packages to uninstall but finally it will finish successfully