Menu bar keeps moving? (when accessing website from different computers)

Does Anyone have any idea why this could be happening?
The menu bar formatting looks perfect on my own pc – but on some computers all the buttons have dropped below the bar!??

Could it be because other computers are using older browser versions? Or do you think resolution could have this effect?

I’m pretty inexperienced so if anyone could give me any advice on how i could fix it i would be really grateful.

Thanks guys!