i cannot transfer files from iTunes to my iPhone, but my iPod works perfectly with iTunes.

I recently changed my PC's CPU (I upgraded to Windows 7), which required me to download iTunes again. while I try to sync music to my iPhone 5 (iOS 7.1.2), it does not happen. neither the "add-to" or "drag and drop" technique works. but when I tried to sync my iPod, it worked perfectly. please help.