Two system(Windows XP and fedora21),how to change the default operating system to windows XP?

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I want to tell you what I am telling anybody using an outdated OS, no matter who the "manufacturer" is: Better stop using it, and upgrade or change it. XP is not receiving security updates and patches since more than a year.

Better would be to run Windows XP in a Virtual Machine. In your position I would remove XP completely, then (if for some odd reason I had to work on XP) I would install it on a VM.