I have read of numerous issues between iTunes 7.2 and Vista X64 so i dont think its fully compatible with the 64-bit version of Vista.Wait for a newer version.I willl search for a FIX and will poet here if i find one
Regarding your PC not shutting down ,make sure you have the latest Windows Updates installed.
Also are you clciking the "Power Button" in the Start menu?
By Default that would Hibernate and not Shut Down.
(In this case go to the arrow to the right of the lock and go to shut down )
Alternatively see if Log Off works , if it does then Log Off and then try and Shut Down (Sounds Lame but may just work
)