Newegg.com is just one of many reputable companies who sell legitimate (legal) OEM copies of Windows. However, note that the OEM license requires you purchase it along with a major hardware component. For instance, a motherboard, a CPU, RAM, a power supply, etc. usually qualify. You can see the store's FAQs page or ask customer service to verify exactly what qualifies. Once purchased it's the same exact OS and full license, with the limitations being you can never transfer the license to another computer or swap out the motherboard.
Hope this helps,
John
Is it legal to buy OEM versions Versions of Vista from Places like New Egg?
Richard

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