Thats a good question. I wonder also. Wouldn't it be possible to just copy it yourself, before the upgrade? Burn it to a CD or even floppy discs would work. I could swear I saw this same topic on Microsoft's web site recently. I do remember someone stating it takes 5 discs. Seems pretty easy to do it that way. Then you can avoid downloading your hundreds of updates when you reinstall. This was a MAJOR pain in the you know what last time I reformatted, and this was about a year ago. There has had to of been 35 or so more updates since then!
As for using your friends disc, the only problem with that, is unless it came from a store, (not the computer manufacturer)
it won't work on your machine. The Op systems usually come with the computer, therefore rendering it useless to anyone who doesn't have the exact same make, model, and brand.
But, if you have a Dell Dimension 4000, or whatever, same CPU, RAM, MHz, and they match exactly, I say use someone else's disc. Why not?