Any ideas ?

[gold_finger]

Could be any number of things going on here, or combo of more than one problem.  Think Scott(0)'s approach to start from very basics is a good way to begin.  Just going to add a few more ideas.

-- Get into BIOS and find where it allows you to restore it to its default settings.  Maybe some things were changed there along the way that are causing problems.

-- Problems booting various CDs/DVDs makes me wonder if the CD/DVD drive itself may be going bad.  If you have a spare to install in the computer, try that to see if it makes a difference.

-- If you do get a live DVD booted, maybe run the Memtest option for a while to check if RAM is okay.

-- Computer might have ability to boot from USB, so maybe make a bootable USB if you can't test a different CD/DVD drive.  Need to have the USB plugged into computer before pressing power button, then need to access the BIOS Boot Menu with whatever keystroke is dedicated to that on power-up to see if it offers choice to boot the USB.  (BIOS Boot Menu is not same thing as the BIOS Settings Menu and requires hitting a different key on startup to bring it up.  POST screen will probably tell you which key to hit, but common ones are <F12> or <Esc> and sometimes <DEL>.)

-- Problems freezing once system is booted could be graphics related, memory related, or CPU temperature related.  You tried a different graphics card and reseated the memory (but didn't do a Memtest yet), so it may be that the thermal paste used to seat the CPU is old and no longer working -- causing the CPU to overheat and thus freeze the computer.  You should be able to get thermal paste from a Computer store or online and replace the old.  Here's a set of instructions for doing that from a manufacturer of a popular brand of thermal paste:  http://www.arcticsilver.com/PDF/appmeth/...d_v1.2.pdf.

Now the screen remains black and the monitor light is red, so the monitor is not detecting the pc is attached to it.

When switched on the power button lights up and stays on unless pressed/held to turn off, all fans spin up, pc not detected by monitor.

Does anyone have any ideas of what to try next, to get it to the stage I am able to get the monitor to realize it is attached and am able to begin instal of LL ?

Connect a different monitor to the computer to make sure that the monitor itself is not broken.  If same problem with diff monitor, then plug original back in.  If problem disappears, orig monitor has bit the dust.

I then tried SATA to usb to find the files on another pc, it finds the hdd when attached to another pc, it says the contents are 0 bytes and produces a message saying the drive is not formatted and asks if I want to format it.

Did you connect it to a Windows computer or a Linux computer?  Connect to a Linux box to see if that makes any difference.  If not, try looking at HDD with GParted and post screenshot of what it looks like.  (In LL, GParted is name of partitioning program found at Menu -> System -> Partition Drives.)