After further investigation, I've realized it's to do with the flash plugin not working, but again only in XP. You can see the code here: http://tarsusempire.co.uk/test/test.html
It works fine in OSX, but not XP. Both browsers are same version and so is flash. I've tried it in FF, Chrome and Safari on OSX, but not at all on XP.
It turned out to be my flash plugin had upgraded itself and then needed for a simple computer reboot. So yes, turning it off and on again resolved this :)