Hi guys, I'm stuck.

For the past three or so hours I've been trying to install the Windows KB2903939 update. At first, it was automatically installed and so I rebooted the computer to complete the updates. Once it reached 100% it displayed the following error:
"We were unable to complete the updates. Undoing Changes. Do not turn off your computer."
I allowed it to reset and booted back into Windows. I tried manually installing the update. Same problem still. I tried running the Windows Update Troubleshooter, it claimed it fixed the problem so I tried it again. Same problem. I tried downloading the updates from the official Microsoft website and installing them from there, same problem. I read online that disabling/uninstalling Avast! May help. Neither worked. Same problem still. So here I am, completely dumb. I have no idea why this small (47MB update) won't simply install. I am NOT wanting to re-install Windows 8.1. It takes too long, and I don't have a disc... I bought my computer with Windows 8 installed and downloaded the free 8.1 upgrade.

Is there ANYTHING I can do to get this stupid update installed without having to re-install everything?
My computer specs are as follows:
Sony T Series Ultrabook.
CPU: Intel i5-3337U at 1.80GHz (2.7GHz with turbo-boost).
8GB RAM
750GB Hybrid Hard Drive (666 GB SATA drive, 25GB Solid State).
Windows 8.1 64-bit.

Frequently Windows updates will get stuck. Try the techniques in this article and see if it helps. Although it is for older Windows versions, the concepts are still valid.

Good luck,
WallyH

This is from the Microsoft community website.

Updates KB2887595 and kb2903939 not updating and causing permission - Microsoft Community


Also some "updates" require admin privliges. Have you tried opening update with admin and then allowing it to install the update?