are u sure that u installed it on a different partition
or a different directory. If you have done the latter
then ur XP installation might have overwritten the ME
setup.
Always install OS in different partitions and not
different folders.
If all you did was install XP in a different directory then you have
essentially overwritten your ME installation. Dual booting requires use of
separate partitions. The MBR is now pointed to XP. There might be a way to
point it back to ME buy good luck finding it.