How to replace lost, broken, or missing Microsoft software or hardware
http://support.microsoft.com/kb/326246/en-us

Signature
Carey Frisch
Microsoft MVP
Windows Shell/User
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Hi, I own a fully legitimate copy of windows XP Professional service pack 2
edition and have come across a problem... during my recent move of home my
windows disk has become cracked. I would like to know how to replace this
disk using my same product key so as not to end up paying full price for the
software again