Ahh, found it...
the key that needs to be added is:
software\microsoft\office\11.0\excel\security
then a DWord called Level with value 1 needs to be added to the key.
if you do this in HKLM, then it'll set everyone's macros security level to low...
Of course, it's generally a bad idea to do this and I realise the correct approach is to digitally sign all code but I don't really have the time to do that at the moment...
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Procrastinate Now!