Boot Camp "Can't install the software" error
I was getting this error as well. In my case the initial download was completing, and then the supremely unhelpful "Can't install the software" error was popping up. There was nothing in system.log to indicate why.

I eventually figured out that it was due to the fact that I was running Boot Camp Assistant as a non-administrative user (for security reasons my day-to-day user is non-admin). Even though, as with most apps that require admin privileges, it asks for an admin password when run as a non-admin user, in the case of this particular app it doesn't work. You must actually be logged in as an admin user. 

Also, make sure you don't have any other users logged in or any other apps running while you are doing the Boot Camp setup, or it may hang at the last minute.

Overall this is a flaky bit of software. When it works it's great, but it copes spectacularly badly with failure. The fact that it insists on re-downloading the support software every time is particularly galling (have these people never heard of rsync?) when you end up running it dozens of times trying to get it to work...