Several small mistakes here:
...var inch = -5
var cm = -5
var mile = -5
...
You must have a semicolon at the end of each statement in JavaScript.
<!-------------------------------------------
You can't have a --sequence within a comment (<!-- -->)
Ok, to display the answer on the same page, you can put a div element like this:
<div id="answer"></div>
and then in JavaScript you can say
window.document.all['answer'].innerHTML = "your HTML tags".
this should actually work with any html tag that can have other tags inside it.
You could let the users select your units from a list of items, using <select>, but that is too easy and the way you let them ask a question is pretty professional, artificial intelligence, cool.
Your script is pretty long and some sections seem to repeat. You could optimise it, etc.
Aligning tables:
I would use one big table with the other tables within them.
It is general knowledge that I can't explain things. So if I failed to make something clear, post and I'll try harder.
