I misread your title, at first thinking it said "Legal compatibility". Which brings me to the point I probably should still make - if you're using such outdated versions in a business capacity for doing things like sending emails (i.e. anything that communicates or reacts with outside entities) and you send/give them a Trojan or virus, you may be legally liable. Obviously that might be country dependent, but it's a major reason why companies here upgrade when M$ stops supporting older versions. Let's face it; many of their upgrades seem to be patches to security holes.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.