Errors 0x800F0900 and 0x800B0100

Good news!

Code:

Windows Resource Protection found corrupt files and successfully repaired
them. Details are included in the CBS.Log windir\Logs\CBS\CBS.log. For
example C:\Windows\Logs\CBS\CBS.log

I ran full scans with MBAM and Kaspersky 2014 (including rootkit scan), but both came up with nothing.

However, this time I deleted the winsxs\...autochk...\ directory before copying autochk.exe from C:\mtsab.

After running SFC, I uploaded the resulting autochk.exe to virustotal, and it matched the good version!

I'm not sure whether it was MBAM/Kaspersky or it was deleting the 'bad' autochk, but at any rate, I'm starting up Windows Update now. Will post back with results, and hopefully mark the thread as solved. Thanks so much for the help!

Edit: marking as solved - SP1 is installing without any errors. Thanks again for being awesome!

Great news! - don't forget to install the new updates that will appear after SP1 is installed.

Good luck!