Because Win9x and WinME are DOS-based, you cannot "naturally" install more than one of them on one hard drive. It is possible, but you will need third-party software like BootMagic (part of PartitionMagic package) or System Commander. These tools will, in essence, hide Win98 from WinME (and vice versa). Multi-booting any versions of NT (NT4, 2K, XP, Server 2K3) and one DOS-based Windows (again, 95, 98, 98SE, ME) can be done "naturally" without any third-party tools.
-SUO, currently multi-boots ME, 2K, XP, and 2K3 (with Linux and FreeBSD for good measure)