SAN FRANCISCO -- Microsoft said Wednesday that its new Windows 10 software is running on more than 75 million computers, tablets and other devices -- in just under a month since the operating system was released.

Analysts say that's a good start, although the company is hoping to get the software installed on a billion devices over the next three years. Microsoft Corp., which is best known for making software for personal computers, is hoping Windows 10 will help it rebuild loyalty among users who are increasingly relying on tablets, smartphones and other devices.

PC sales have been shrinking in recent years and that trend is likely to continue, according to a new forecast from International Data Corp. Analysts at IDC predicted Monday that 2016 will mark the fifth consecutive year of declining PC sales worldwide.

Computer-makers are hoping to get a boost from consumer interest in Windows 10, which manufacturers are making available on new machines being sold this fall. But IDC noted in its report that Microsoft is also offering the new software as a free download for people who want to install it on their existing PCs.

The free upgrades probably account for the bulk of the 75 million devices now using Windows 10, said analyst Patrick Moorhead of Moor Insights & Strategy, although he said more new models will likely be sold this fall.

Some of the machines getting Windows 10 are quite old. Microsoft executive Yusuf Mehdi, who released the new figures Monday, said in a Twitter post that, "even some devices manufactured in 2007 have upgraded to Windows 10."

Along with new features, the Redmond, Washington, software giant designed Windows 10 to fix some unpopular elements of Windows 8, the operating system's last version. It's intended to work on PCs, tablets, smartphones and other devices, although the company released it for PCs and tablets first.