'spinnerlys', as I already said, there aren't that many viruses for a Mac, but there are various ones in different flavours, and some of them will infect the system on their own if you stumble on a webpage with the right one or stupidly click on ads and popups (however, most viruses, evenm those for Windows, require user interaction to run ... because when you come down to it, users are interacting with the Internet all the time, so the danger of picking up a virus or malware rests on them).
My own cerebral cortex is rather comfortable with Windows and I do not really encounter issues.
Heck I also never experience degradation of the OS speed over time (from XP to 7 because I know how to run basic maintenance and hot to keep the thing clean - most of which is unknown to a lot of people who use computers).
I tried Mac OS and wasn't really impressed, plus it didn't really provide me with anything that I cannot do in Windows (furthermore, Mac created more problems because some programs I use are non-existent for Macs ... and to top it all off, the price tag of a Mac is simply too much and is downright stealing if you ask me - I couldn't care less about aesthetics of a laptop or a desktop, I care more about just how powerful the components inside the machine are and what they can do, while offering that power at a reasonable pricetag - not to mention the fact I found dozens of PC laptops and desktops that have a much more inviting design in contrast to the Macs, and their functionality was exceptional - but, that's just my experience and opinion).
Now, let's get back to the original question of this topic, which I think went something like this:
Is it possible to have a machine running two accounts and only have an anti-virus actively on one?
Man has bought second machine round which he mainly wants to use for gaming. He has given me an account on it and wants me protected (and his machine!) as I will be compelled to come here and other nefarious places. Apparently though a resident anti-virus will slow down the running of the games, so I wondered if any of you had any idea if this could be done?
The answer is 'yes'.
From what I understood, you are saying that your 'man' will have 2 accounts on the computer (one for himself and one for you).
If that is the case, all he has to do is install the antivirus on his own (administrator) account and then create your account (with or without administrator priviledges).
Most, if not all of the programs from the administrator account would be vissible/accessible in a guest account (or a second administrator account), plus these days, programs usually ask you if you want to install a particular program for just yourself or everyone who uses this computer (other accounts).
I recommend choosing ONE from the following 3 programs (all of which are free, highly effective in protecting the computer and do not reduce performance in any capacity - there is simply 0 need for a paid antivirus these days):
Microsoft Security Essentials, Avast 5 (free) or Avira 10 (free).
Man has got a great machine with a huge graphics card, a big black flat screen and a totally horrendous old white keyboard ~ I mean aesthetically, it is not pleasing 
