Your quickest solution is to try another free PDF reader:
Best Free Non-Adobe PDF Reader
My guess is that your Adobe installation went awry and is preventing a re-install. If you still want to use Adobe, download Revo Uninstaller and use it to uninstall any remnants of Adobe. Then try...
For starters, go to desktop and press Windows Key and R and type in msconfig. Look for Startup tab and disable all programs. Close and restart your system. If that cures the problem then it is one of those programs you disabled. For more on the msconfig tool just Google it.
Since there are no responses, I'll give it a try by including this from the WD site:
Microsoft Windows 64-bit Operating Systems and WD productsAnswer ID 1430 | Last Updated 10/09/2014
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External Hard Drives
Our external hard drives should work with 64-bit...
Uninstall both programs, reboot and re-install programs. Would also recommend Scan Disk and SFC.
If you have an optical drive you might try booting with an ISO downloaded from here:
Create installation media for Windows 8.1 - Windows Help
Unless one of our gurus comes up with other suggestions, you may have no other alternative than to reformat and do a clean install.