help fixing my imac

I have a friends imac that i am trying to fix, but all the tricks i know aren't working....help!!

Here is what is happening... the computer is taking ages to start up about 10 minutes, it gets to the enter password bit, i enter that and its works fine but after that I just see the desktop picture and nothing else...

I have tried to reboot holding down various keys but nothing seems to happen.

Help...please

Did you accidentally deleted system files or renamed a user? Since when do you have this problem? Can you give more information about the OS?

Tell us exactly which Imac it is and what OS version is on it.