Hi guys what was the build of Longhorn used to demo areo at WinHEC 2003. I think it's Build_4015.lab06 but i'm not to sure,

Yes, 4015 Lab6, that build shown at Winhec is unleaked so far.

And, I don't know wether it is actually true or not, but from what I have heard, it doesn't include the sidebar.

It propably had just sidebar disabled because of the memory leak...

For just a refference, i've posted the WinHEC03 4015.lab06's demonstration long time ago here (YouTube).
We also have article about 4015.lab06 with screenshot and photos at our wiki...

I took it with a pinch of salt, (Meaning I was a bit suspicious in this case), but I was told it doesn't even include the sidebar, because that special version of Build 4015 was use for demoing prototypes of the DCE, they couldn't be doing with the memory leak issue with the sidebar, (like you just said), getting in the way of the demo. Imagine how embarrassing it would be if it crashed like that Windows 98 beta demo Bill Gates did when he was demoing the pnp capabilities by plugging in a scanner!