I just bought a G3 blue and White tower second hand and everytime I turn it on it's getting to the main screen with all the icons along the bottom (this is my first mac) then it is either crashing back to a blue screen then coming back to main screen or coming up with a message telling me I need to restart or it's going on to a black screen and asking for a login and password? It keeps varying between the 3, I want to wipe everything off and start fresh but have no idea how or how to stop it crashing? Please help!