Spoke too soon, it's now running at 80% but it does not appear to be doing anything other than slowing the computer down.
Edit: We are on EPO server 4.6, not 4.5.
I would probably take a look at what other programs I have installed on that machine and this sounds like a possible clash happening between McAfee and other services ( read software) or may be incompatibility.
Could you please check the On Access Scan logs and inform if you find anythng there ?
Probably the Windows Indexing service ?
Which other process consumes CPU time when McShield does (in Taskmanager) ?