want to fresh reinstall your mac from a usb key 
if your updating your mac from the itunes store download diskmakerx from here http://liondiskmaker.com/
start the download of from itunes and plug in at least a 8 gig usb key
when the download finishes it will want to start the install, before you start the install run disk maker and it will find the install image downloaded for maverick and then proceed to find the usb key and confirm you want to use it, when you select this is fine you wait around 10 / 15 mins and you have a bootable usb key with maverick ready to go.
Pop into you mac / imac and turn on with the alt key selected which will offer you the choice to boot from the usb !! nice fresh install 🙂