Ah... Been there, done that
Make sure you have this in writing from the big boss:
You can get admin access if you absolutely want it.
But if something (no matter how trivial) gets fucked up on the PC it will be a full wipe/re-install. IT won't attempt to trouble-shoot as there is no telling what you have messed up.
Make clear how long a full re-install is going to take.
And make absolutely sure that he himself is responsible for backups of any local data on the PC.
Also make sure they are aware that on any sign of network trouble (virus outbreak) his PC will be the first suspect and be subjected to a time-consuming full virus-scan from clean boot-media.
It may be worthwhile to give them a scare some week or 2 after getting admin rights: Slip a false positive in the temporary internet files. When the virus-scanner goes berserk confiscate the PC for a full virus-scan and mutter something along the lines of "I hope I can clean this... I don't really have the time for a re-install.. You do have a recent backup of your data I trust ?"
Of course: If the CEO or company owner doesn't back you on this you are screwed. Still, inform them, in writing that you consider this a major bad idea. If only for the "I told you so" opportunity when things do go wrong. (It usually will not take long for that to happen.)