I assume you mean your hard disk is 150GB and there is only one single partition.
If your partition is on NTFS file system, then you can't. Windows ME cannot boot from an NTFS partition.
If it's FAT32, it is possible, but it's not a good idea to install 2 OS's in the same partition.
If you don't want to lose what's on the XP partition, you should get third party software to create another partition for you first, format it with FAT32 and install ME there. Then use a boot manager to dual boot.
You can try Partition Magic which has Bootmagic as its boot manager.