If you want to see it in action. Put an unpatched Windows XP system straight on the internet with no firewall, service packs, antivirus and a real IP.
How long should I keep it on for? At what time could I turn it off and say this test is done? I have a spare laptop. I can install Windows Xp (Not even SP1!), nothing else, and plug it directly into the cable modem.
Given an infinite amount of time, an infinite amount of monkeys can write Shakespeare, apparently, but what if they had a timeframe? The test is impossible, because if I leave it for a day, or a week, You'll say I didn't leave it on long enough.
Do me a favor, don't put words in my mouth.
Just give it a night, but before you do this, you should lookup how long the average infection time is for an unprotected system on the internet. You should have conficker or MSBlast in no time.
Honestly, I don't care if you think a virus can infect you or not, just because you are an individual. I know they can infect your machine without you doing anything to get them. It's happened to me. Just because something doesn't happen to you, doesn't mean it doesn't happen.
At peak it was something like 20-30 seconds! (you didn't even have time to do online patching of the machine) Spread was IIRC pretty much exponential, every infected machine would try to infect other machines with similar IP addresses etc., nasty, nasty stuff if you weren't behind a proper firewall. Probably less these days because XP share is much reduced, and most people actually have the service packs etc. installed. Still a few idiots do hold out, connect their machines to the internet and act completely oblivious to the problems they're causing. You didn't have to do anything to get infected, just connect the machine to the internet and leave it, in many cases you wouldn't even know you were infected unless you monitored your outgoing traffic.
Anyway, no, if you're not going to be actively patching a machine then you shouldn't have it connected to the internet, period.
Actively patching something like a MAME cab could introduce other problems if some of the patches break functionality that certain frontends rely on, or even in some cases MAME (see the number of people having problems with that crappy hacked up shooter build based on 0.99 and full patched Win7 64-bit boxes)
Therefore I conclude, cabs are best kept offline.
I wonder how many commercial games with internet connections, running Windows based OS's are actually infected with this type of crap (I'd hope the embedded versions don't have the exploited services enabled, but you never know)