Upgrading the Computer always requires Re-installation of Windows.?

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HI friends I am about to replace my system with the upgraded version, Presently i am using dell optiplex gx260. now i am planning for the same brand dell gx 280. But i am curious for the attempt of re-installation of windows as the dealer told me i will have to re-install the windows xp in order to upgrade my pc,because i have too many programs installed which i will have re-install all over again. Is there anyway that prevents re-installation prior to system upgradation smoothly.?