NEM wasn't booting for me either (only the spinning mouse cursor spinned for a bit and then nothing, as nzminer said), but I guess it was the antivirus (avast) blocking it: as, testing, I have disabled avast for 10 minutes, and NEM loaded normally.

Now I have added an exception on the avast config, excluding NEM folder from the antivirus protection, and it's working perfectly.

Good that I didn't have to redownload everything again.