...then all you need do (as Grif suggested) is make a copy of the file to another internal hard drive or to CD/DVD.
Here's what I do: I download everything to one folder (on this Vista system, that's C:\Users\Paul\Downloads). All installation keys (including the ones from CD/DVD install disks are entered in a Word Pad file I title "Software Serial Numbers" that I keep in C:\Users\Paul\Documents. In that file, I also enter all pertinent data for this system (serial number of the PC and all hardware I've installed into it, etc.). I then simply copy the C:\Users folder to a second internal hard drive and to a DVD. NOTE: C:\Users\ also has all our audio/video/picture files as well. The DVD is kept off site (at work) - just in case of a real disaster (fire, flood, etc.).
This does take a while to set up, Stephen, but once set up it takes less than an hour/week to update.
So, how to begin: Assemble all your disks and keys. Enter the keys to the Note Pad file and save it. Copy all your install disks using any one of a number of available programs. Copy all your downloaded installers to another CD/DVD.
Should you have issues retrieving keys, try two pieces of freeware: The Belarc Advisor available at http://www.belarc.com and the Magical Jelly Bean Keyfinder available at http://www.magicaljellybean.com/keyfinder/. With these, you should get most of your keys back. If you're still missing some, perhaps the maker of the software can assist you in retrieving the others.
Paul