Q: mac late 2007 crashes after boot. latest OSX El Captian update

Hello Dom, 

I understand that you are having issues with your Mac starting up. Let's go over some troubleshooting options to get this working again. 

I would start with booting up in Safe Mode. When powering up your Mac, hold the Shift key until you see the Apple Logo and progress bar. Once you see that you can let go to see if it boots to your user. if it does, then you can restart your Mac and it will startup normally.

If it still crashes, you will want to move to using Disk Utility to verify your startup disk. For a more detailed look at what to do, take a look at the article below to get you started. 

Resolve startup issues and perform disk maintenance with Disk Utility and fsck
https://support.apple.com/en-us/HT203176

​Take care