It won't be available for another four months, but consumers are already rushing to place their orders for Microsoft's highly anticipated Windows 7 operating system and take advantage of early-bird discounts.

Windows 7 screen shot
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Amazon and other retailers have been taking pre-orders for Windows 7 since June 25th. For the third week in a row, the OS is Amazon's top seller in the software category, according to the Web merchant's Best Sellers list.

As of Tuesday, the list showed Windows 7 Home Premium Upgrade as the top seller in software, followed by Windows 7 Professional Upgrade. Microsoft Office 2007 was ranked third, completing a win-place-show trifecta for Redmond.

In keeping with a Microsoft promotional campaign, retailers—including Amazon and Best Buy—are offering Windows 7 at discounts of more than 50% until July 11.

Windows 7 Home Premium Upgrade, for instance, is selling on Amazon for $49, or more than half off its suggested retail price of $119. Windows 7 Professional is going for $99, down from $199. That's good news for Microsoft, which saw Windows sales fall 16% in the most recent quarter.

Microsoft said it would defer recognition of 50% of revenue for sales made through the program until the fulfillment date of the purchases or until the program expires, based on whichever comes earliest. Microsoft said it expects to defer $200 to $300 million in such revenues during its fiscal fourth quarter, which ended June 30th.

Microsoft has also disclosed an upgrade program through which consumers who purchase a Vista-powered PC can migrate to Windows 7 at little or no cost when the latter becomes available on Oct. 22.

HP and several other PC makers have said they would offer free Vista-to-Windows 7 upgrades.

InformationWeek has published an indepth report on Windows 7. Download the report here (registration required).