i can't get iTunes to recognize my iPhone.

I can't get iTunes to recognize my iPhone. I want to sync my music library but iTunes can't see my phone. I have run through all the diagnostics available from Apple help. I have the latest version of iTunes on my computer (PC running Windows XP). A few months ago I was successful syncing my music with the same hardware. The cable for my iPhone is good. I know this because I can move photos to my computer. Please help. I have tried everything. I just purchased more music via iTunes from my computer and I want to be able to listen to it from my iPhone.