Honestly, the simplest solution seems to be the one that isn't garnering much attention.
You need to explain to the head of IT (Probably through your immediate supervisor if you have one), that scans at noon are killing your PC's performance, and making a negative impact on your productivity. Seems to me that if the scans ran at end of day, or prior to the start of work hours, your primary issue would be taken care of. It's not particularly difficult for even a modestly competent shop to make that happen via Wake on Lan, and a few scripts. Working in IT for years, I've encountered control freaks, inept admins, and genuinely helpful people - I try to be a member of the last group. If the anti virus system is managed via a server console, they should be able to remedy the problem without even returning to your desk.
I generally try to enable my users the ability to have as much flexibility with their systems as I can. Messing with anti virus software is one place where I keep their hands off - for numerous reasons. Running a full scan at noontime is just a bad plan, and should be pointed out as such to the powers that be.