Mail on Imac

I bought an Imac 2 days ago and to begin with Mail opened fine. Now it will not open and instead when I click on Mail I get "Welcome to mail" message and it tells me I have to import message, once I click to import "preparing to import message" appears which goes on forever and nothing happens.

Shortly after setting Imac up 2 days ago there was a new software upgrade which I downloaded and I think it's since then MAIL hasn't worked.

Have tried downloading the same software upgrade again but after an hour of downloading it now says "the Mac OS X Update Combined can't be installed on this disk".

Has anyone had a similar problem and how did you resolve.

Thank in advance for all suggestions.