Hi, In my experience, I've found this the best place to come for answers, so this isn't linux related 100% but I'm hoping you could at least point me in the right direction.

Whenever I try and install an OS (or start windows 8 from disk) from CD or USB, I'm getting a form of IO error. On linux mint it is Errno 5. A bit of research reveals answers, but none fix it for me. Windows gives me 0xc00000e9. SMART data reveals nothing more than "OK". Tried lots of partition combinations, formatted in all sorts of ways, I just can't fix it.

Googling the windows error code gives this: http://answers.microsoft.com/en-us/w...a-8e2a99d67b80...seems hardware related

I've seen that. What could I do about it except change the hard drive?

'smart' data is a good starting point, but if the error persists across various operating systems, then it is most likely as jkirchner mentioned, hardware related and the only way to 'fix' said hardware is to replace it, though before doing so i would do a more thorough test on the hard drive, processor and ram, as any of those could create an i/o error.

Test your RAM with Memtest86+.
Test your disk(s) with the manufacturer's diagnosis tool.

How would I do that?

Use a different computer to download the Memtest86+ ISO from here: http://www.memtest.org/#downiso
Then burn it to a CD like you would with a Linux ISO and boot your computer from it. The test will start automatically, let it run for a few hours. If it doesn't show you red error messages your RAM seems to be OK.
Then use a different computer to download your disk manufacturers diagnosis tool, it will also be a bootable ISO (some manufacturers give you the choice, in that case make sure to download the DOS version to get an ISO). Boot from that and follow the on-screen instructions.

memtest can be downloaded here (http://www.memtest86.com/) (burn the iso to a blank cd or dvd)
the manufacturer's diagnostics tools can be downloaded from the manufacturer's website, burned to a disk
you then boot from the disk and follow the instructions.

Done e mem test before, no problems

Here are the final diagnostics from my computers diagnostics.
CPU = Pass
Memory = Pass
Drive (Hitachi..., my hard drive) = Pass
CD drive = Pass
Boot Path = Warning, No bootable drives detected.

Please don't use the diagnostics from your computer, they are very basic. Please use the tools we recommended to you, they make much more reliable tests.