iPhone 3G will not sync - no error messages

Hi, I cannot get my iPhone 3G to sync on either of my two PC's (both running XP). I have iTunes 9.0.1 installed. I have uninstalled iTunes on one machine, rebooted the PC and installed iTunes again - still no joy. No message appears on iPhone or on PC. Have rebooted iPhone several times using soft key and on/off key. Have rebooted PC. Both PCs have "services.msc" started. Under devmgmt.msc, device is not seen under USB Controllers. Have tested USB by connecting iPod - which is immediately detected.
Please let me know what else there is to try...