Microsoft probably would support a legacy system, but for the right price. A couple of posts to the WSUS forum recently for one of the nastier security vulnerabilities seemed to indicate folks still have agreements in place for NT 4.0 support. That was EOL a while ago. I remember talking with our TAM about that when NT 4.0 was finally going to be dropped unless you had a support contract explicitly for that purpose. The cost to maintain were rather high.