Hi there and welcome Guy
What is the exact warning you get? And in which program is this occurring in?
ok, in which program is it happening in Guy?
I can see you are a first time user to the site. What we do here is either a question and answer type of conversation or remote support where I would actually connect to the computer and have a look and attempt to fix the problem for you.
Remote support can save a lot of time if you are not comfortable performing repairs yourself. Sometimes even with instructions you can make a mistake and make things worse.
I will send an offer through for remote support and should you accept I will connect and see what is going on and fix the issue. Otherwise we can continue to talk like this but I will need to ask a number of other questions and try and walk you through some steps.
Both options are 100% guaranteed by the way. Remote is just quicker and avoids the risk of you making a mistake and doing something wrong.
If you want remote click get extra help. If not simply reply to me.
yes but its optional, else we continue here.
When did this problem start? And what did you install just before it occurred?
Power off the mac
Then start the Mac and hold down the Command and R keys at startup.
Hold these keys until the Apple logo appears. After your computer finishes starting up, you should see a desktop with an OS X menu bar and an OS X Utilities window
choose Disk Utility from the Utilities window, or from the Installer menu if you're started from an installation disc.
Click the First Aid tab.
Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
Select your Startup Disk (usually named "Macintosh HD").
Click Repair. Disk Utility checks and repairs the disk.
Let this complete and when it has, tell me if you still have the problem please.
You return to this web page and post a message, like you are doing now. The message will then come straight back to me Guy
Ok, do the below
And tell me how it then runs please
ok, its possible your computer is just too under specifications for El Capitan.
Download and install Google Chrome from my link below and tell me how that operates please
and how is it operating?
ok, then your going to have to clear this by doing a full reinstall. this is the case when you have these issues and the repairs fail.
First, copy off any data from the Mac (programs you will need to install again). Then
Once it's done, quit Disk Utility and get ready to reinstall.
Once done it should then function correctly.
Let me know the results please