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I have had the same problem with my iPhone 3GS and my 2nd gen iPod touch. I have a Toshiba Satellite L300 (32-bit) and Windows Vista. Have tried uninstalling and reinstalling iTunes and its components a number of times. I followed every single step in the Apple support forum for this topic- restarting/repairing the driver, making sure Apple Mobile Device Support is set to automatic, try to manually locate the driver and it says "the best software for your device is already installed". So frustrated as I paid $100 for Apple Care Plus and when contacting apple I was essentially told there was not a lot they could do since I live no where near an Apple store. I believe the problem is with the Apple Mobile Device driver but for some reason it won't update. Help! (Oh, and I've tried on other computers, same problem)
The problem is in fact the driver.
1. Right click "computer" and choose properties
2. Select Device Manager from the left pane.
3. Find your phone in the list of devices.
4. Rich click and choose "update driver software"
5. Select "Browse my computer for driver software"
6. Select "Brows" again.
7. Navigate to the following directory
C:PROGRAM FILES (X86)COMMON FILESAPPLEMOBILE DEVICE SUPPORT
8. Click "Next" and wait for it to replace your driver.