Shillingworth's right. It's just not possible to make the old UI work now like it did back in the day. It simply doesn't function the same way.
With tabbed windows, separate chats, larger banks, hard-coded window elements, etc, etc ... you just can't do it and please the people that want it exactly the same.
The best you could do is make a whole new UI using elements and styles from the old one that is compliant with today's code. It wouldn't be the same, but it could have that nostalgic feel and look.
Personally, I don't miss that old cramped UI one bit. The
only thing I do miss is the old hitsmode settings and the way they'd scroll off the screen and fade away, heh. That was very nice, and didn't require a separate chat window for filtering.
Edit -- Here's a link to one that's been made and is still updated, I think, if you wanna see how difficult it is to do, heh ...
LINK