If you're dual botting, you have to install the operating systems in order of their age. For example, the first step is to partition your hard drive. Then install Windows Me on one partition. Then install Windows XP on a second partition as normal.
Doing this should automatically set up a boot manager for both operating systems. In case it doesn't follow the following steps from within WinXP. Similar steps can be used from within Win98.
In Windows XP, right click on My Computer and go to properties. When System Properties opens up, go to the Advanced tab and then click on Startup and Recovery. Where it says "Default operating system" click to edit the boot.ini file manually. Make sure there is an entry for both operating systems you have installed. In addition, select a default OS. In addition, select how many seconds (default=30) you should have to choose an OS before the default OS is automatically loaded.