Hi guys. I know this is an old old thread, but seeing as it in the top google results for searches on this issue I thought I'd share a simple solution.

After running the appropriate .reg file found on this page Default File Associations - Restore in Windows 8, do the following. (direct .bat link here)

Open regedit.exe

Go to HKEY_CLASSES_ROOT\batfile\shell\open\command

Set the default value to the absolute path of cmd.exe followed by /C "%1" %* - my value looks like C:\Windows\System32\cmd.exe /C "%1" %*

Now open up Control Panel > Programs > Default Programs > Associatiate a file type or protocall with a program

Find the entry for .bat and click change. You should see the option Windows Command Processor. If not, set it to something else first and do it again - I found it may not show the registry defined option at first.

DONE. You can now run .bat files and you didn't have to reinstall. Microsoft seem to have done something very strange here, and decided to set up their file associations somewhere other than the registry - though to fix things you need to make registry changes!? Who knows. I know the above works though!