i have a iphone 3g and am running windows xp my phone wont sync with computer what can i do

For the iPhone 3G, the latest iOS it can run is 4.2.1, so if you have that version, that is as far as you can update.

If it is not connecting to the computer, you can check this support document http://support.apple.com/kb/TS1538

Do you have the latest software updates for Windows and Apple software installed, including QuickTime?

More suggestions here:

iOS: Device not recognized in iTunes for Windows