Itunes not picking up Iphone?

Help, my itunes isn't picking up my iphone..

It been happening for a while back before Christmas but I didn't think much of it i thought it would sort itself out. But today is my upgrade to the new iphone 4s and i don't want to have the same difficulty with that...

Basically i plug my iphone(3g) into the computer with itunes running (latest version) my iphone makes the little ding sound to say its plugged in, and my computer (windows) makes the dong sound to say a USB has been plugged in, but then nothing.... i get the option from windows to copy items from the device but that's it nothing else, my phone says its charging and it is, but itunes doesn't pick it up at all, it stays the same as if it isn't plugged in, and it doesn't start a sync. I haven't synced ( that word sounds weird) my iphone in a longggg time maybe 5 months because of this and i really need to so i can save my data for my new iphone and also i don't want to get my new iphone and then stupid itunes not pick it up so i cant set it up.

Oh the frustration...

Anyway if anyone can help me that would be great, like i said its an iphone 3g, i have the latest itunes, the computer and iphone are picking each other up but not itunes, its been happening for about 5 months, getting a new iphone and don't want the same problem.

Thank you to anyone who can help me out, much appreciated.