iMac login problem please help new mac user

[Xdelboyx]

Hi guys bit of a weird one I just recently bought an iMac 2008 and it has developed a weird problem when I got it it had snow leopard but I upgraded it to mountain lion straight away it did boot up fine for few days no problems however it has developed a problem where by it will boot all the way to the textured graphite grey user profile screen where you choose what profile you want to open I have 3 there normally one is mine one is my wife's and the other says guest but now it gets to that graphite screen and does not display any user profiles just the blank screen with my mouse icon I can however login to each different profile because I know where on the screen they usually sit...my wife's one is in dead centre normally so if I click dead centre even tho there is no image or anything there it will log in my wife's profile I can then log out back to graphit screen still blank and click centre and slightly to the left again with no image or anything an it will login my profile it doesn't effect the day to day running of the computer but still I would rather it worked correctly I can boot by holding option control p and r and it displays them but next time I shut down it is back again also have disconnected my external USB hard drive in case it was bad peripheral and still the same it does have an apple USB keyboard and apple USB mighty mouse and I will try booting without these connected when I get home tonight to see if it is a problem with one of those devices ...I hope it's nothing major and you guys can help me thanks guys