Just found an old CD-RW with files on it, including a yahoo messenger executable ?

Nope.  Unless you learn coding, even if you had the source code you wouldn't know what you're looking at.  Even hackers who steal and crack software need to know coding inside and out.

So, double-click the executable and find out for yourself if it does something useful.

It's still copyrighted software, though, so it won't really be "your own version" even if you do hack it up.

No, You need to understand the coding.

You could open it up in hex editing software, then translate it to assembly, but chances are you need more than the final exe to make any sense of the programming

yes all you need now is a computer that has windows 95 to run it .

Yes, there are many thousands of people who already know how to reverse engineer such things and you could simply hire one of them for your "own version without having to reinvent the wheel or learn coding" yourself.

Not that hard.

Also not clear why anyone would want such a thing, given the 20 years of improvements developed since that one was designed.

If you're not going to do any coding, then you'll have to use it as it is.  So, double-click the executable and find out for yourself if it does something useful. 

It's still copyrighted software, though, so it won't really be "your own version" even if you do hack it up.

The honest answer is that if you have to ask this, you're not close to ready to disassemble a program.

Reverse engineering an executable without having to learn coding...!

If you had even remotely the required skill level you'd never ever say something as ridiculous as that.

not easily. you would need to understand the machine code.

and honestly, learning to write actual code would be easier.

sorry.