Imac will not update past 10.6.8

Hello,

My imac is currently running on 10.6.8. I continue getting a notification that i have a software update. i click to install it and it successfully downloads and asks to restart to complete installing. I hit restart and the computer starts booting but gets stuck on the purple galaxy color screen which is totally blank. i hold the button on imac and shut the computer down and turn back on. When it comes back on nothing is changed and it tells me an update is available. I went into the app store and downloaded el capitan which failed at the end of the download and wont install. My imac is eligable for el capitan. I can no longer update some programs or use new ones i download so any help to fix this problem is appreciated.