Well I've now found the point in the log where it fails, and it smells suspiciously like 'Everything'!
MSI (s) (FC:C4) [19:10:55:213]: Executing op: ProgressTotal(Total=27754556,Type=0,ByteEquivalent=1)
MSI (s) (FC:C4) [19:10:55:213]: Executing op: SetTargetFolder(Folder=C:\Program Files\JPSoft\TCC21\)
MSI (s) (FC:C4) [19:10:55:213]: Executing op: SetSourceFolder(Folder=1\)
MSI (s) (FC:C4) [19:10:55:213]: Executing op: ChangeMedia(,MediaPrompt=Please insert the disk: ,,BytesPerTick=65536,CopierType=0,,,,,,IsFirstPhysicalMedia=1)
MSI (s) (FC:C4) [19:10:55:213]: Executing op: RegisterSharedComponentProvider(,,File=Everything.exe,Component={CCE4A56F-5A5C-46EF-9CFC-8C972D4A1A66},ComponentVersion=1.4.1.877,ProductCode={FCB877BA-BC4E-40DD-91A8-3166F856DB21},ProductVersion=21.1.47,PatchSize=0,PatchAttributes=0,PatchSequence=0,SharedComponent=0,IsFullFile=0)
MSI (s) (FC:C4) [19:10:55:217]: Executing op: FileCopy(SourceName=EVERYT~1.EXE|Everything.exe,SourceCabKey=Everything.exe,DestName=Everything.exe,Attributes=0,FileSize=2197608,PerTick=65536,,VerifyMedia=1,,,,,CheckCRC=0,Version=1.4.1.877,Language=1033,InstallMode=58982400,,,,,,,)
MSI (s) (FC:C4) [19:10:55:217]: File: C:\Program Files\JPSoft\TCC21\Everything.exe; To be installed; Won't patch; No existing file
MSI (s) (FC:C4) [19:10:55:217]: Resolving source.
MSI (s) (FC:C4) [19:10:55:217]: Resolving source to launched-from source.
MSI (s) (FC:C4) [19:10:55:217]: Setting launched-from source as last-used.
MSI (s) (FC:C4) [19:10:55:217]: PROPERTY CHANGE: Adding SourceDir property. Its value is 'C:\Windows\Installer\'.
MSI (s) (FC:C4) [19:10:55:217]: PROPERTY CHANGE: Adding SOURCEDIR property. Its value is 'C:\Windows\Installer\'.
MSI (s) (FC:C4) [19:10:55:217]: PROPERTY CHANGE: Adding SourcedirProduct property. Its value is '{FCB877BA-BC4E-40DD-91A8-3166F856DB21}'.
MSI (s) (FC:C4) [19:10:55:217]: SOURCEDIR ==> C:\Windows\Installer\
MSI (s) (FC:C4) [19:10:55:217]: SOURCEDIR product ==> {FCB877BA-BC4E-40DD-91A8-3166F856DB21}
MSI (s) (FC:C4) [19:10:55:217]: SECREPAIR: CryptAcquireContext succeeded
MSI (s) (FC:C4) [19:10:55:217]: Determining source type
MSI (s) (FC:C4) [19:10:55:217]: Note: 1: 2203 2: C:\Windows\Installer\tcc.x64.msi 3: -2147287038
MSI (s) (FC:C4) [19:10:55:217]: Note: 1: 1316 2: C:\Windows\Installer\tcc.x64.msi
MSI (s) (FC:C4) [19:10:55:217]: SECREPAIR: Error determining package source type
MSI (s) (FC:C4) [19:10:55:217]: SECUREREPAIR: SecureRepair Failed. Error code: 524EBF044B8
Error 1316. The specified account already exists.
MSI (s) (FC:C4) [19:10:59:503]: Product: TCC 21 -- Error 1316. The specified account already exists.
I selected NOT to install it, so why does it still try? "MSI (s) (FC:C4) [19:10:55:217]: File: C:\Program Files\JPSoft\TCC21\Everything.exe; To be installed; Won't patch; No existing file"