Hi Iain,
I see that this is your first post in this forum, welcome to the Microsoft Community forum.
Just a few questions first, to understand the issue better:
1) What is the full error message that you get?
2) Do you remember the KB number or any information about the update that was installed?
From what you have described, it appears that the issue is caused due to a conflict with the update that was installed recently. Since you are unable to boot to the desktop now, I would suggest you to perform an automatic
repair and check. The Automatic repair can detect and fix the issues that prevent the computer from starting normally. You will need the Windows 8 installation disc to perform an Automatic repair. If you don’t have a Windows 8 disc, I would suggest you to
download the Windows 8 installation files from another computer through this link and choose to create an installation media when the upgrade assistant prompts.
http://windows.microsoft.com/en-US/windows-8/upgrade-product-key-only
http://windows.microsoft.com/en-US/windows-8/create-reset-refresh-media
After you have a Windows 8 installation disc, follow these steps:
a) When your computer starts, wait for the manufacturer logo to check the option for boot menu, it will usually be F12.
b) The boot menu key might differ with each manufacturer. If you are unable to find the boot menu key, you may refer the user manual.
c) Restart the computer, when the manufacturer's logo is displayed, keep pressing the Boot menu option key to enter the boot menu and change the boot option to CD\DVD
ROM.
d) After you boot your computer using Windows DVD or System Repair Disc, a black screen appears with gray text "Press any key to boot from CD or DVD". Press any key.
e) Select the correct time and Keyboard type.
f) Click Repair your computer in the lower left corner
g) Click on Troubleshooting, advanced option and then click Automatic repair.
Try these steps and let us know the results. We’d be happy to help if you need any further assistance.