A: A Service Pack is essentially a collection of updates, so if Windows Update isn't working, it's likely a service pack will also fail. I received an error related to RPC_S_CALL_FAILED, which typically indicates problems communicating back to Windows Update, possibly caused by a firewall blocking (so check your firewall). However, in my case it was simply the problems with Windows Update, which was confirmed by looking at the Application event log. It showed the TrustedInstaller having repeated errors. To enable the installation of the service pack, you need to first fix Windows Update, as outlined in this FAQ.