A server box can coexist SQL 6.5 ,SQL 7 and SQL 2000.You should first install SQL 6.5 then latest Service pack for 6.5 --> SQL 7 then latest Service pack for 7 -->SQL2000 Then latest Service Pack for it.
You can have one SQL 6.5 running
You can have one SQL 7.0 running
You can have multiple instance SQL 2000 running,SQL 2000 can run as named instance but not for 6.5 and 7.0
You can switch between 6.5 and 7.0 by SQL SErver Switch