Microsoft's highly anticipated Windows 7 operating system became available for pre-order for the first time Friday as several online retailers touted the software at deep discounts. Early sales appeared robust.

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

Windows 7 Home Premium Upgrade, for instance, is selling on Amazon.com for $49, or more than half off its suggested retail price of $119. Windows 7 Professional is going for $99, down from $199. Amazon is also offering Super Saver Shipping on Windows 7, which will be available on Oct. 22. Best Buy is offering free shipping.

Consumers appeared anxious to take advantage of the discounts, which run through July 11. As of early Friday, the OS was Amazon's most popular product in the software category, according to the Web merchant's Sales Rank report.

That's good news for Redmond, which saw Windows sales fall 16% in the most recent quarter.

In a further effort to boost sales, Microsoft on Thursday 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 has said it would offer free upgrades.

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 current fiscal fourth quarter, which ends on June 30th.

Microsoft shares were off slightly in pre-market trading Friday, down .71% to $23.62.

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