Ahar! Now I think I know where your problem is. Many of the most popular files available are from T3 sources & most of these are corrupt. So that's why you can't preview them. The RIAA is flooding T3's with corrupt files to stop people's enthusiasm.
Check these threads out:
http://www.gnutellaforums.com/showth...threadid=28801 http://www.gnutellaforums.com/showth...560#post112560 http://www.gnutellaforums.com/showth...3195#post93195
My advice is to avoid T3 sources where possible.