There's additional potential for confusion because there's one version of Windows 8, called "Windows RT," that looks like the PC version but doesn't run regular Windows programs. It's intended for tablets and lightweight tablet-laptop hybrids.
Budiu believes the transition to Windows 8 will be most difficult for PC users, because Microsoft's design choices favor touch screens rather than mice and keyboards. Alex Wukovich, a Londoner who tried Windows 8 on a friend's laptop, agrees.
"On a desktop, it just felt really weird," he said. "It feels like it's a tablet operating system that Microsoft managed to twist and shoehorn onto a desktop."
Not everyone who has tried Windows 8 agrees with the critics.
Sheldon Skaggs, a Web developer in Charlotte, N.C., thought he was going to hate Windows 8, but he needed to do something to speed up his 5-year-old laptop. So he installed the new software.
"After a bit of a learning curve and playing around with it a bit more, you get used to it, surprisingly," he said.
The computer now boots up faster than it did with Windows Vista, he said.
Vista was Microsoft's most recent operating-system flop. It was seen as so clunky and buggy when released in 2007 that many PC users sat out the upgrade cycle and waited for Windows 7, which arrived two and a half years later. Companies and other institutions wait much longer than consumers to upgrade their software, and many will keep paying for Windows 7. Many companies are still using Windows XP, released in 2001.
Colin Gillis, an analyst at BGC Financial, is optimistic about Windows 8, pointing out that it's snappy and runs well on PCs with limited processing power, making it suited for compact, tablet-style machines. But he also notes that through Microsoft's history, roughly every other operating-system release has been a letdown.