Help! stuck on "Setting up your Mac"

I started to install Mavericks this afternoon at around 1pm. I previously had Snow Leopard 10.6.8 on my Mac Pro (which I just recently aquired, with this pre-installed. I had an old PM G5 before it).

When we left the house at 2pm it was still on "Setting Up Your Mac" screen. We've just come back about 10 minutes ago and it's still on that screen with no signs of doing anything else.

Is this a common issue? How can we fix this?

Thanks