All the search results I found for that error seem to be about having trouble installing SP1, but this machine is already running SP1 (and has all updates installed except the one thats failing, KB2778344).
I tried quite a few things, renaming the software distribution folder so it would download it fresh (this helped a little but, there was initially 3 updates failing and this let me install two of them), running the fixit to reset/repair windows update, running the system update readiness tool, using the windows update troubleshooter in control panel. I also scanned for malware with malwarebytes which found 2 items in the user's internet explorer temp files and removed them, didn't find anything else and shows no other symptoms of malware.
edit: update readiness tool gives me these errors in the log:
Unavailable repair files: winsxs\manifests\wow64_microsoft-windows-win32k_31bf3856ad364e35_6.1.7601.22137_none_21e50706feb2618b.manifest winsxs\manifests\amd64_microsoft-windows-win32k_31bf3856ad364e35_6.1.7601.22137_none_17905cb4ca519f90.manifest winsxs\manifests\amd64_microsoft-windows-win32k_31bf3856ad364e35_6.1.7601.17977_none_16dba817b1543c48.manifest winsxs\manifests\wow64_microsoft-windows-win32k_31bf3856ad364e35_6.1.7601.17977_none_21305269e5b4fe43.manifest servicing\packages\Package_2_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_2_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_3_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_3_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_for_KB2761226_SP1~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_for_KB2761226_SP1~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.mum servicing\packages\Package_2_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.cat servicing\packages\Package_2_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.cat servicing\packages\Package_3_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.cat servicing\packages\Package_3_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.cat servicing\packages\Package_for_KB2761226_SP1~31bf3856ad364e35~amd64~~6.1.1.2.cat servicing\packages\Package_for_KB2761226_SP1~31bf3856ad364e35~amd64~~6.1.1.2.cat servicing\packages\Package_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.cat servicing\packages\Package_for_KB2761226~31bf3856ad364e35~amd64~~6.1.1.2.cat







