Mac Erased?!

I logged onto my iMac 21.5" this morning to find that everything (almost) is erased! When I say almost, I mean that all of my applications are there and my documents (that I can see) but when I go into iTunes for example, it gives the greetings that you get on a brand new computer. This is very strange and I am concerned that it may be a virus. I have been using MacKeeper and I may have went over board with deleting stuff but i don't think that this could affect the files this way? Also on a side-note, Mail won't let me send a message.

It's not a virus as there are no viruses for the Mac. And, MacKeeper is not going to allow you to delete any vital files. Do you remember exactly what you deleted or cleaned with MacKeeper?

Are you currently using Time Machine to make backups? If you are (and you should be using it) go back in time to where your iMac was running normally and restore.

You didn't happen to rename your Home folder, did you?

Did you install any updates for iTunes recently? And when you opened iTunes did you still have all your playlists, playcounts etc?