From an M$ press release:
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We figure we spent about $100 million incrementally on that particular analysis of both the shipped versions and what we could do to remediate some of the things that we felt were still not good and then making new design choices for both the .NET Server product, which is finishing its development cycle now and is in test and then ultimately for things that could be fundamentally changed in a subsequent implementation of the product, which is now known as the Longhorn release that's several years hence.
...
So we know that in practice it's impossible for us to remediate the threats that we know exist in the world today in systems that were designed in 1991, '2 and '3 and deployed in '95 and which are actively still in use today.
...
both because it affects the design of the next version and it allows us to remediate some of these things even for the shipped version
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What, pray tell, is wrong with remedy, or just plain "fix"?