iPod touch not recognized in iTunes for Windows

Check the iPod touch Display
When you connect iPod touch to your computer, the iPod touch should display an image or message on the display. If you do not see any image, refer to support document #305740 for troubleshooting issues where the iPod touch won't turn on. In some cases, you may see a message on the screen that says "Charging... please wait." This indicates that the iPod touch does not have sufficient battery charge to operate and will need to be charged for several minutes before it can be recognized in iTunes. In other cases, you may see a message that indicates that your iPod touch requires service. In this case contact AppleCare or visit the Mac Genius desk at your local Apple Retail Store.

Verify Windows PC System Requirements
iPod touch requires iTunes 7.4 or later. Be sure that your Windows PC meets the minimum system requirements to use iTunes and iPod touch. This is particularly important if you are connecting iPod touch to your computer for the first time. To make sure you're using the latest version of iTunes, open iTunes and choose Help > Check for Updates.

Check USB Connections
iPod touch requires USB 2.0 to function properly. Refer to the documentation for your computer to verify that the USB ports are high-power USB 2.0 ports. Try connecting iPod touch to a different USB port, preferably a port that is built-in to the computer instead of an external USB hub. Connecting iPod touch to your keyboard's USB ports is not recommended.

Restart your PC
In some cases, your computer's USB ports may become temporarily disabled or you may experience other software related issues. Restarting your PC is an easy troubleshooting step that may resolve this issue.

Verify iPod touch Driver/Service Installation
iPod touch requires two important pieces of software to operate properly on Windows. First, the driver must be installed; for iPod touch the driver is USBAAPL.SYS. Second, Apple Mobile Device Support must be installed as a Windows Service. If these are not installed, see the Reinstall iTunes section below.
Verify that the driver is installed by connecting iPod touch to the computer and following these steps:

1. On the Start menu, click Run.
2. In the Open field, type devmgmt.msc and click OK. A list of devices will be displayed.
3. Scroll down to Universal Serial Bus controllers and click the small plus sign to expand this group. iPod touch will be listed as either Apple iPod touch or Apple Mobile Device depending on the version of the driver installed.
4. Right-click Apple iPod touch (or Apple Mobile Device) and choose Properties from the shortcut menu.
5. When the Properties window opens, select the Driver tab.
6. Click Driver Details to view more information.
   Verify that the Apple Mobile Device Service is installed and running using the Services MMC snap-in using these steps:
7. On the Start menu, click Run.
8. In the Open field, type services.msc and click OK. A list of devices will be displayed.
   View the list In alphabetical order and verify that the "Apple Mobile Device Service" status is "Started."

-If the status does not indicate the service is started, right-click Apple Mobile Device Service and choose Start from the shortcut menu.
-If the service is installed and started, right-click the service and choose Restart from the shortcut menu.
-If the driver is installed and the service is installed and started, check the Event Viewer for error messages.
-On the Start menu, click Run.
-In the Open field, type eventvwr.msc and click OK.
For more information on viewing or interpreting event logs in Windows, see Microsoft Knowledge Base Article #308427, "How to view and manage event logs in Event Viewer in Windows XP."

Reinstall iTunes
Make sure you are using iTunes 7.3 or later. Apple recommends using the latest version of iTunes. You can check the version of iTunes you are using by choosing About iTunes from the Help menu. Visit the iTunes download page at www.apple.com/itunes/download/ to download and install the latest version or choose Help > Check for Updates in iTunes.
While you do not normally need to remove iTunes before reinstalling it, in some cases this is recommended. If you see a message stating that "The software required for iPod touch is not installed. Run the iTunes installer to remove iTunes, and then install iTunes again," then iTunes should be removed before reinstalling. For more information on removing iTunes, refer to support document 93698.

Source: www.apple.com