The two updates are: Cumulative Security Update for Internet Explorer 10 for Windows 7 Service Pack 1 for x64-based System (KB2898785) and Internet Explorer 11 for Windows 7 for x64-based Systems.
I run Norton Internet Security, it's up to date, and I've done a full scan since trying to fix the problem. I've run Malwarebytes Anti-Malware scanner and Anti-Rootkit scanner. I've run ESET Scanner yesterday - it found some instances of Toolbar.Conduit.H which have been quarantined.
If someone is willing and able to help, I'd be extremely grateful. Having worked in IT security for a large part of my working life, I certainly don't like being unable to apply a security update - sadly my knowledge has been growing rusty for a couple of years now!