I've discovered interesting extra information about my 1542 errors.
Windows detected your registry file is still in use by other applications or services. The file will be unloaded now. The applications or services that hold your registry file may not function properly afterwards.
DETAIL -
1 user registry handles leaked from \Registry\User\S-1-5-21-989875291-3784188841-2934234737-1000:
Process 10260 (\Device\HarddiskVolume2\Program Files (x86)\Common Files\microsoft shared\Source Engine\OSE.EXE) has opened key \REGISTRY\USER\S-1-5-21-989875291-3784188841-2934234737-1000
Event 1530
I've discovered I have multiple instances of OSE.exe on my system even on other drives (I have four drives)
On F: (not my system drive) it is here: F:\MSOCache\All Users\90000409-6000-11D3-8CFE-0150048383C9\FILES\SETUP
On C: (mysys drive) it is here: C:\Program Files\Microsoft Office 15\root\vfs\ProgramFilesCommonX86\Microsoft Shared\Source Engine and here:
C:\Program Files (x86)\Common Files\microsoft shared\Source Engine
Seems very odd?