I have the same problem on two computers running Windows 8.1 x64. Both give exactly the same error message in Event Viewer (except for the 'User', obviously).
They both get to the end of the update and then say failed. I have tried using the stand alone installer as well. Here is the output from Event Viewer from one of the computers:
Log Name: Setup
Source: Microsoft-Windows-WUSA
Date: 19/11/2014 19:41:17
Event ID: 3
Task Category: None
Level: Error
Keywords:
User: DESKTOP\P
Computer: Desktop
Description:
Windows update "Update for Windows (KB3000850)" could not be installed because of error 2147942402 "The system cannot find the file specified." (Command line: ""C:\WINDOWS\system32\wusa.exe" "F:\Windows 8\Windows8.1-KB3000850-x64.msu" ")
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Microsoft-Windows-WUSA" Guid="{09608C12-C1DA-4104-A6FE-B959CF57560A}" />
<EventID>3</EventID>
<Version>0</Version>
<Level>2</Level>
<Task>0</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000000</Keywords>
<TimeCreated SystemTime="2014-11-19T18:41:17.951334800Z" />
<EventRecordID>3208</EventRecordID>
<Correlation />
<Execution ProcessID="2140" ThreadID="3224" />
<Channel>Setup</Channel>
<Computer>Desktop</Computer>
<Security UserID="S-1-5-21-518983977-3308070022-825916067-1000" />
</System>
<EventData>
<Data Name="UpdateTitle">"Update for Windows (KB3000850)"</Data>
<Data Name="ErrorCode">2147942402</Data>
<Data Name="ErrorString">The system cannot find the file specified.</Data>
<Data Name="CommandLine">"C:\WINDOWS\system32\wusa.exe" "F:\Windows 8\Windows8.1-KB3000850-x64.msu" </Data>
</EventData>
</Event>
Any help please?