My iPod Touch 5g stopped showing up in iTunes on my Windows 8 PC. My wife's iPod Touch 5g was workign just fine though. I went as far as to completely wipe my iPod back to factory settings and even then it wouldn't show up. I also kept getting the "Trust This Computer" message every time I'd connect. After trying pretty much everything here, I searched for info on the trust message. I found a page that showed the steps to find Apple's USB driver. Even though I had tried updating my driver through Control Panel and was told it was up to date, as well as uninstalling my iPod through Control Panel and reinstalling it, the Apple driver finally did the trick.
Below are the steps that worked for me. To give credit where credit is due, I got the steps from http://helpios7.com/trust-this-computer-pop-up-fix/
1. Go to Control Panel > Hardware & Sounds > Device Manager
2. Right-click the Apple device > Properties > ‘Hardware’ tab > Properties > ‘Driver’ tab > Update Driver..
3. Search for the driver in “C:\Program Files\Common Files\Apple\Mobile Device Support\Drivers”
4. The driver should be named ‘usbaapl64′
I didn't personally need step 4. After I did step 3, my only option was to let Windows search that directory for me, but after that, I unplugged the iPod and plugged it back in and now I can finally go to bed.
I hope this info helps someone else out there.