keynote update

I've been trying to update the keynote on my computer for a few weeks. I have removed it, intaled it again, but it won't update to the newest version. The same happens to Pages. The instalation starts, the bar gets moving foward, than it goes back to zero. The process starts over once more, and the I get the message: "un error has occured", and "use the purchuased page to try again". Any one has a clue what I should do to fix this?

Thanks