Anything i need to know? Will i need to re-install windows? Apart from the obvious, is there anything i ought to know that might cost me my entire machine if i mess up?
It's really easy. Just disconnect everything and reconnect them with the new PSU. You can't really mess up. The worst thing you can do is forget to connector something and the computer won't boot properly. And no you do not have to re-install windows.
Just ensure that the psu thats going in your machine is upto the spec required and its good a quality one, nothing worse than thinking that your fitting a powerful psu only for it to blow the whole pc, but yeah should be no probs with it just remember where you unplugged from and re-connect
As for the installation, its the easiest part in building a computer. Basically mount the PSU on the tower then connect and you're ready. Thinking on what PSU to buy is harder than actually installing it