I think your safest bet is to attach a large capacity external USB drive and simply copy all of the data over

That is certainly the simplest way. It also has the advantage that the data is accessible on other machines.

Proper backup programs should be quicker and usually have the advantage of being able to save the computer's settings - accounts, browser bookmarks/favorites etc. Which may or may not matter to you. In the case of NTBACKUP (I used it on XP and now on Win7) you can restore a backup taken on XP onto Win7 if you replace the machine. (And probably on other Win versions too, but that combination I've checked.) Other backup programs may not be able to do that.