Use Time Machine to backup itunes. Then download a new copy from Apples website and install itunes again.
Note: If you are having issues with iTunes or other applications, you should first run Disk Utility. iTunes library content such as music, videos, and iPod games are saved in your Music folder by default and are not deleted by removing iTunes or its associated files and folders.copied from
http://support.apple.com/kb/HT1224if you can connect to another computer, you can back up manually on this one. Deselect autosync in itunes, connect your phone and right click on it in the device list and choose backup. You could restore from that backup if you need to in case someting goes wrong.
MBP 17 2,16GHz Mac OS X (10.5.7) iMac 2,4Ghz, ipod 5G, iPhone3G, ipod touch 2ndGen