As Ankur said its illegal and its also a security risk to download from unofficial sites. Everybody or a common user may not be able to recognise what that download contain. He may not check whether its digitally signed by MS etc.
The danger from downloading from such sites is who knows what they might have added/modified to it? It may just drop a malware/trojan along with it. ANd people may start blaming MS. So unless you are sure its from authorised/ Official site dont download from such unofficial sites.
If you are sure its clean and the source is trustwothy, you can try it . Just search on the Net and you'll get it.
And if you want to test such upcoming products from MS, register at Connect. http://connect.microsoft.com/