Probably because the one in XP takes about a million years to defrag a large drive and offers no real options such as the ability to pick files and folders to defrag. This is especially important if you edit a lot of audio and video.

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99 ML320
94 SL600
92 SL500
95 E320 Cabriolet
87 560SEC
86 300SDL Grease Car
80 380SLC Euro

13 Fiat Abarth
02 Maserati Spyder Cambiocorsa
00 BMW Z3
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80 Ferrari 308 GTSI
88 Jaguar XJS12 H&E Edition
99 Land Rover Discovery