Common problem. It's because Nokia doesn't have a quick-access button to disable packet data. Here's the manual way to do it:
1) For WLAN(wireless internet) problem
try this:
Menu>settings>Configuration>personal settings>options>add new>
choose "my access point", "my streaming" & "my web"
Then
Menu>settings>Connectivity>WLAN>Internet **bleep**.>Ask First
Menu>settings>Connectivity>Packed Data>>pack. data conn.>When needed
Last
Menu>Internet>Web Settings>Configuration Settings>configuration>Personal configuration
2) To watch Youtube via WLAN
1st make sure U go to configuration settings
see personal settings, make sure u create "My streaming"
after that, U open media player (the one that play your songs)
click options>more>Downloads>streaming Settings>
change...
configuration>personal config.
Account>my streaming