i am wondering if i buy fallout new vegas on steam without downloading then get a new desktop set up the same account on it will i be able to download fallout nv onto the new desktop or can i only download it onto my old one.

Games are tied to your account not your computer. So long as you don't have steam online with your account on both machines, you should be fine.

please explain

Don't go online with steam on your account on both machines at the same time.

ok thanks

To be more precise, you can download the game on both computers - and probably even play it on both at the same time. But you can't go online on your Steam account on two computers simultaneously.