How to Unsync the iPhone
The iPhone automatically synchronises to your iTunes program when it is connected to your computer, but if you use two different computers the alternate computer cannot synchronise content without first writing over your iPhone's original information.
Fortunately the iPhone has a manual synchronisation setting that allows you to choose what to sync to your device, which allows the iPhone to be used on several computers. If you want to turn off auto-synchronisation for other types of information including contacts, calendars, mail, and photos, you must manually disable the auto-sync settings in the iTunes program.
Run the iTunes program on your computer, and connect the iPhone to your computer through the mobile phone's USB cable.
Click the name of the iPhone in the side bar on the left side of iTunes, and click "Manually manage music and videos."
Click the "Apply" button to save your new synchronisation preferences, and click the "Info" tab in the main iTunes window.
Uncheck the box next to "Sync Address Book contacts," and the box next to "Sync selected Mail accounts." Uncheck the box next to "Sync iCal calendars," and click the "Photos" tab at the top of the iTunes window.
Uncheck the box next to "Sync Photos from" to disable automatic photo synchronisation.
Click "Apply" in the lower-right corner of iTunes to save the changes. Click the "Eject" button next to the name of your iPhone to manually disconnect it from the computer.