I downloaded it once, doing a "SaveAs" to a file name and location on the desktop. I then copied the file over to my laptop. I then opened them for the first time.
Opening each and running them as needed has been flawless for half a year now.
Edit: Whoops. I'm not answering your question. I'm talking about the free PDF.
To your point. I've purchased the NECH on CDROM in previous years. Haven't gotten the 2011 Handbook yet. They have all come with two installs. I had to rebuild a system and re-install, a call to NFPA gave me additional installs.
I'd think the download is the same. A bit is a bit on the computer.