Closing as an antivirus issue. I'm assuming the AV is removing the EXE files as a false positive.
The node-webkit preview mode launches <install path>exporters\html5\node-webkit\win32\nw.exe. If there is still an error in preview, check that file still exists. If it is missing, reinstall Construct 2 with AV temporarily disabled.
You may also need to temporarily disable AV when exporting your project. We need to do this when building Construct 2 itself, since antivirus sometimes tries to check EXE files half-way through the write to disk, which breaks the write to disk, and makes the file corrupt or missing. I'm not sure what we can do about this since it's a simple file copy, and we already work around it ourselves by disabling the antivirus temporarily, so I guess that's the thing to do.
To reduce the chance your end-user's antivirus flags your published game, digitally sign the .EXE file (another thing we do with C2).