Nope, no previews before the dld starts.
If you take the time to think about it, it's rather fairly simple and logical: you can't preview something you don't have and unless you start dlding the file, then you don't have it.
It's almost as if you asked: "Can I eat an apple without having one?"...
It's not impossible to do it, but it would need streaming and to do that in P2P would probably slow down everyone.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-