Hi sub,
Its good that you are evaluvating MS Dynamcis, but unfortunately you will not be able to download it, it is accessable only from a partner source(you have to be a partner of Microsoft). however you can see some flash files and movie clips from the internet at www.microsoft.com/dynamics/nav/product/demos.mspx.
if you would like to evaluvate Nav, you have to get the help of a microsoft partner, We are Gold Certified Partner for Microsoft and i would be glad to help you out.
Write to me at krishna.prakash@adea.com / mailkprakash@gmail.com (gtalk) for further informations.
Cheers
KrishnaPrakash