If you have installed two windows in your computer i.e. Windows XP in C drive and Windows 7 in D drive. As and when we start computer, by default windows 7 automatically starts. In case you want to set start computer via windows XP by default. With the help of following path in Wnidows 7 , default setting can be changed :-
My Computer >Property > Advanced System Setting > Advanced > Setting of Sartup and Recovery >Change Default Operating System.
Picture view of setting is given as under :