Windows 7 is the fastest-selling operating system in history, giving a boost to sales of new PCs. But only 10 percent of existing Windows Vista and XP users have upgraded to it. That’s why Parallels is launching software today that makes it dead simple to upgrade to Windows 7.

The Parallels Desktop Upgrade to Windows 7 software walks users through easy-to-use instructions on how to upgrade software safely. It shows you how to upgrade without losing data or having to reinstall programs. It does the latter with a system that will automatically install your existing programs on Windows 7 and then enter the necessary certification codes to make them run.

Microsoft ought to be grateful for this. A survey that Parallels did showed that nine out of 10 people worried about problems relating to upgrading to a new version of the operating system.

“We make it simple enough so that you don’t need to be a Windows 7 expert,” said Peter Bauert, chief operating officer and chief financial officer of the Renton, Wash.-based company, in an interview. “The virtualization is seamless to the user. It happens automatically and silently.”

Normally, you have do a clean install — wiping out your data and programs and then reinstalling them from scratch — in eight of 15 different installation scenarios. With Parallels, you don’t need to do the clean install. You can use Parallels to upgrade from Windows XP or Windows Vista to any version of Windows 7.

To install it, you put in your Parallels CD and click on the right option. Then you put in your Windows 7 disk and enter the authentication code. After that, it’s all automatic. If you need to roll back the installation, you can do so. It costs $39.95 for the software, or $49.95 if you want a high-speed universal serial bus cable to speed the transfer of data from the old computer to the new one. There are discounts for five or more copies of Parallels. The software will be available today on the company’s web site and on May 31 in retail stores. It is available in English, French and German.

Parallels uses its virtualization software to get around a lot of typical problems. The company already has 10 million customers in 125 countries. It also has 500 partners, including Microsoft. It is the No.1 desktop virtualization company, with 70 percent share of the market.

Parallels was founded in 1999 and is now a $100 million-plus company in annual revenues. It has more than 700 employees and is profitable. Rivals include VMWare, LapLink and Microsoft.

In 2005, Parallels raised outside money for the first time, with $12.4 million from Bessemer Venture Partners, Insight Venture Partners, and Intel Capital. At the time, annual revenue was just $7 million.