Bootcamp touchpad drivers fail to install on Windows 8 RTM
Go to Device Manager and uninstall (and delete drivers) for the two
  Trackpad devices then rescan for hardware. After which, then go find
  your Boot Camp drivers, and don't run the full setup, but instead
  find the folder that specifically has the driver install packages in
  them and install the ones that are along the lines of "Apple
  Multitouch Trackpad." After this, go to your Boot Camp options to make
  sure that it now sees your Trackpad.   You should have full
  functionality restored after this.


Did get the answer from this forum post: How do I get my Macbook Pro trackpad working on Windows 8