Can I just install windows 8 preview alone?

yes.

go download the ISO file and burn a bootable dvd to install for the OS on a new computer

I can just put the installation file on a usb drive then install it from boot?

yes.
http://hexus.net/tech/tech-explained/software/31765-how...

a better explanation.

Is there a timer on it? If so is it untill launch

It will expire, but not sure when.

Do i need a key from another copy of windows
it asks for a key

, Microsoft should have sent you one when you chose to download the o/s.
Check your email or go back and try and download again and use that key.

The key is listed here: http://windows.microsoft.com/en-US/windows-8/iso

January 15, 2013.

http://www.jkwebtalks.com/2012/03/windows-8-consumer-pr...