In the UK it's spelt with an "s" mate ...
ROFL!! Georgina, I love your style. being an Aussie, I have the same issue. I feel compelled to use 'S', but then no-one (including google/live) know what you're talking about!!
So, I've resigned to the fact that when I'm talking Windows, I use a 'Z' - and then be a rebel and use 'S' everywhere else!!
I think we should just be a rebel and use "s" all the time. I might event go crazy and spell System Center...wait for it....."System Centre"!!!