Hey,
So my fathers Lantra is best described in one word as poo. Ok so I have three problems here, and I'm ready to burn the thing! Would appreciate if I can get some help.

OK! Here is problem number one:

All doors but one will open, the front passenger door won't open. How can I get this door open to take off the door panel to see what the problem is? Also with the doors, all my lock's are screwed, drivers door won't unlock with keyless, you need to use the key simply because its locked all the time, one door stays locked all the time and you can't open it, the other door locks itself, but won't unlock unless you use your finger, and the last door won't lock at all and when you go to lock the door from the driver's side, you hear a grinding noise coming from that door.

Problem number 2: ALL my windows are broken! They are all powered windows and somehow the cable managed to come off the rollers and pulleys, they are all just tapped up!

Last problem, my stock head unit, sometimes (most of the time now) none of the buttons work. I have spoken to my mechanic about these problems and he has no idea about the first two, but knows that 98 to 2000 model Lantra's had terrible wiring harnesses.

A conclusion to these problems would help. Thank you.

Sounds like you got some ugly problems there. On your door lock problem... since the remote doesn't work and I take it the inside button(s) don't work I would think that one or more buttons might be faulty or stuck in the lock position (electrically) preventing the remote from working. Try disconnecting the master door lock control and then try the remote. If no luck reconnect it and disconnect the lock buttons on all the other doors and try the remote again. If this works reconnect the locks 1 at a time. Hopefully you will narrow the problem down.
As to the door you can't open... you might try a "slim jim" and fish for the lever or take off the drivers side panel and study the mechanism carefully and practice with the slim jim. I don't think you can remove the panel without opening the door. even if you take out the seat I think the dash will still be in the way.