Both accounts are identified as 'computer administrator'.
The only difference I can quickly establish is that the AntiVirus 'Enable Script Blocking' is ON for the computer with the 'Access Denied' issue, whereas the AntiVirus version on the other computer has no such option?
I tried installing with the 'Autoprotect' disabled, but still got the 'Access Denied' warning. Presumably that made no difference because nothing was being actively downloaded from the Internet (I'd unplugged the connection to the router).
Would selecting 'Enable Script Blocking' OFF be likely to make any difference?