Saves won't open on one computer but will on another

I have recently gotten a new computer and installed minecraft. I can create a new world but once I quit that game, I can't re-open it (both survival and creative) but I can still see them. When I click on the single player option in the main menu, I can see the worlds but they aren't highlighted when I click on them and the "Play Selected World" button doesn't get highlighted or work. The only buttons that do work are "Create new world" and "Cancel". I still have the old computer (windows vista) with minecraft which I can play the saved worlds on (copied via USB stick) and create and play new worlds with no problems. I have tried rebooting and re-installing minecraft but it hasn't changed. I also tried uninstalling and re-installing Java with no change.