Macbook Pro Booting Screen issue after upgrade to El Capitan

I have recently upgraded to El Capitan on my MacBook Pro (15-inch, Late 2008), 2.66 GHz Intel Core 2 Duo, 4 GB 1067 MHz DDR3 and everything was working fine for two days. On the 3rd day however I get this screen issue when starting up and nothing happens.

I then have to reinstall El Capitan and it works again for two days, again on the 3rd day I get the same issue.

Any help would be greatly appreciated.

Thanks