When I connect my iPod to my computer, iTunes display a message, saying that: this computer is no longer allowed to use applications on my iPod. I need to enter my credentials for iTunes Store at each session to be a be to sync my Apps.
First disconnect your iPod/iPhone.
Go to C:/programs/iTunes.
Right click iTunes (Application) > Properties > Compatibility.
Check "Run this program as administrators".
Open iTunes and log-in to the Appstore.
Reconnect your iPod/iPhone and sync your applications ..