How to use downgrade versions of SQL 2008R2 Ent. Edition

Hi,

Currently we are using SQL 2000 std. edition in the environment, which is not geniun. Before 2 years we purchase SQL 2008R2 Ent. Edition (Volume License). My existing database structure will not support SQL 2008R2. So we need to continue SQL 2000.

We want to use geniun product so Kindly suggest from were I will get SQL 2000 std geniun key & ISO for further use.

Thanks.