WindowsME does not allow Real Mode DOS execution. Therefore, in order to successfully run BeOS 5 Personal Edition with WinME, you must create a BeOS boot disk.
The same applies for Windows NT4/2000/XP. To create a new boot disk, insert a fresh floppy and run c:/beos/CreateBootFloppy.exe. When the process is complete, reboot your machine with the floppy in its drive.
If you already have a working BeOS partition elsewhere, you can run makebootfloppy from within BeOS rather than the Windows equivalent.