Question : sql server 2000

Is SQL Server 2000 still supported, i.e. in terms of security, are they still patching security issues found? What is the difference in terms of security updates for mainstream support and extended support?

Answer : sql server 2000

It means "paid for in full"... Academic is out.  Express (MSDE) out as well.
It does not matter when you bought it, it is the lifecycle of the product (i.e. SQL Server 2000 w/ SP4), not your license.

The plan was 10 years, but since standard support was extended to 2008, the extended goes 5 years beyond that.