I downloaded the full ISO for Visual Studio Ultimate CTP 6. The installation program got to about the 90% mark, gauging by the progress bar, and just stuck there. There was frequent activity from Superfetch, Anti-malware protection, and other background processes, but the progress bar was dead still. Eventually the background task activity subsided after 20 minutes, but the progress bar still wouldn't budge.
CHEAP TRICK: Open a notepad window and position the left edge of it so it perfectly marks the current position of the progress bar. If it the progress bar doesn't move past the left edge of the notepad window in about an hour, it's probably stuck.