Most old software gets remaindered to the bargain bins. Not Windows XP, however. In June, Dell wangled a deal with Microsoft to let it install the older operating system for customers who didn't want Vista. In June, the companies charged $50 extra. According to this order page, XP now costs an extra $99 — on top of the cost of Windows Vista, which is baked into the basic price for the computer. Here's the full order page:
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MS sees XP Pro as the predecessor to Vista Business, and XP Home as the predecessor to Home Basic. XP Pro even has more features than Vista Home Basic (encryption, faxing, etc).
Source: ERM Blog