Ideally you need a clean install of WIndows 8, just an fyi, trackpad is not working after an upgrade only. On rMBP trackpad works like a charm after the clean install of W8,
Saw a soluton here:
Follow these steps after you have upgraded from Windows 7 to Windows 8 on a Mac book Pro. I had the same problem that tracket was not working.
Download and keep power plan assitant, trackpad++ from this website
Step 1:Go to Device Manager and on the two "Touchpad" entries with the bang on them, went to "Update Driver" -> "Browse My Computer" -> "Let me pick from a list..." and changed them both to USB Input Device. Reboot.
Step 2: Unistall in case you install Trackpad ++ initially and it was not working. Similarly uninstall power plan assistant.
Step 3: Reboot
Step 4: Do a clean installation first of power plan assistant and then of trackpad ++ that you have saved as installer. So both these are installed now.
Step 5:Go back to Device Manager, do an "Update Driver" again, let it search automatically, and one comes back as Apple Multitouch and the other as Apple Multitouch Mouse.
Step 6:Rebooted again, trackpad working and shows up in Boot Camp control panel.
TRUST ME IT WILL WORK !. I struggled y'day 10/27 after my Windows 8 upgrade and installation when trackpad was not working on my MBP. But the above suggestions helped and I am back working on Windows on my MBP.