Global sales of Microsoft's new operating system, Windows 8, has been slow at best but devices running the cross-platform OS were moving in a much better pace, tech blog site Mashable reported this week.

The report dampened earlier claims by Microsoft chief executive Steve Ballmer that the new product he is pushing hard for the whole world to discover, backed by a marketing war-chest of around $US1.5 billion, has reached download counts of four million in its first 96 hours of availability.

The feverish push, it seemed, for the redesigned Microsoft core offering failed to draw the anticipated attention, if the Mashable report was to be believed, despite the rallying of celebrated names behind the product.

Popular talk show host Oprah Winfrey, hip-hop superstar Jay-Z and movie actress Jessica Alba were among the global luminaries that lend their star-appeal for the OS that Microsoft dubbed on one of its numerous commercial campaigns as 'Everything at Once'.

Citing a blog post penned by Paul Thurrott, Mashable said on Monday that Windows 8 "isn't selling as well as Microsoft thought it would ... the new OS isn't even meeting lowered expectations."

The assessment, Mr Thurrott claimed, was reportedly provided by an unnamed but 'trusted Microsoft insider' and was pegged with data on earlier major tech product debuts.

Reasons reportedly discussed by Microsoft officials for Windows 8's poor sales performance so far were the inability by the tech giant's partner PC vendors to deliver as per the specifications strictly set by Microsoft for hardwares to be paired with the OS.

This, Mashable said, led to confusion on customers' part and likely slowed down the product's movement.

It was also believed that the recent Microsoft leadership shakeup, which saw the immediate exit of Windows chief Steven Sinofsky, hampered what should have been expected as brisk sales for Windows 8, Mr Thurrott wrote in his blog.

Another indication of Windows 8's not-so-good sales to date was provided by SlashGear, which reported of "a huge demand for the product that never materialised," with the tech blog site quoting Newegg executive Merle McIntosh.

However, the same Newegg official also informed SlashGear that sales of Windows 8-affililiated products - hardwares and softwares - were "steadily improving."

Mixed reviews have so far greeted Windows 8 as experts branded the OS as initially confusing owing to its radically overhauled interface but potent enough to support both mobility and productivity.

In particular, Microsoft's Surface tablet, which runs on another version of Windows 8 - Windows RT, has been getting positive feedbacks, with one review hailing the slate as the best productivity tablet to come out.