I want to reinstall Windows, but my computer doesn't have a recovery partition and I don't have the OS CD/DVD. Is it legal to download the same version of Windows I had installed, burn it to a DVD, and install it using my own product key found underneath my laptop?
Nah, no big deal. If the software cops kick down your door and you have a legitimate license key for all the software that you're using, you're fine, even if you've got seedy media.
Make sure you download the OEM version: OEM keys don't work with the regular version (for XP and older.)
Technically, no, it's not legal. The act of downloading the files would be a copyright violation. But this is so minor it's one of those cases where I wouldn't let that stop me, and I'm usually a stickler about copyright issues.
I'll also note that there's a distinction between downloading the files and using the files after you've downloaded them. The latter portion is perfectly fine, even though the files were first obtained illegally, because you're complying with the license terms in how you're using everything. So if you already have files, even if obtained illegally, or if you can just borrow someone's disc (which would mean you never make an illegal copy) every thing should be just fine.
That said, copyright law is incredibly complex (imho one of the big problems with it), and so this is just my interpretation.