After entering the information of the first time and syncing for the first time, iTunes will create a back-up file with all that information just incase. This is the case for both Mac and Windows.
So for instance, if it's a new PC, then iTunes will start to "sync" with your iPhone and will copy any apps (if you have your iTunes account registered in that new PC), contacts, calendar events and such.
iTunes will NOT delete your stuff unless you checked the "Replace data on the iPhone with data from this computer"