Take an underpowered, limited- capacity Mac. Load it up with full versions of Microsoft’s Windows and Office software. Now try running everything at the same time, including the Mac’s operating system, without crashing or slowing things to the speed of the Long Island Expressway on a summer weekend.Parallels Desktop 7, the latest version of Parallels Inc.’s Windows-in-a-Mac software, successfully navigated my little torture test with nary a hiccup. There’s no better solution for Mac users who want Windows compatibility and don’t mind spending $80 extra (plus the cost of Windows itself) to get it.You don’t need Parallels to run Windows on a Mac. Ever since Apple Inc. (AAPL) switched to using the same Intel Corp. (INTC) chips that run most Windows personal computers, it has had a free program called Boot Camp that lets you switch back and forth.Boot Camp, though, has several limitations. You have to choose which operating system you want to start up, and reboot if you want to go back and forth. And there’s no easy way to share files between the two sides, or to transfer information from one to the other.Parallels allows the two to co-exist, letting you switch between them seamlessly, even cutting and pasting between applications. You can run an entire PC in a window on your Mac desktop, complete with Windows 7’s “Aero” user interface with its translucent borders. Or you can choose a mode called Coherence that hides Windows, making PC programs feel and behave almost like native Mac applications.To try out the software, I used a year-old 11-inch MacBook Air with a now two-generations-old Intel Core 2 Duo processor, four gigabytes of memory and 128 gigabytes of storage. I had previously installed Windows and Office, using Boot Camp to run them when needed.I started out using Parallels with the Snow Leopard version of the Mac’s OS X, then upgraded to the latest version, Lion. The installation process under Snow Leopard went smoothly, with one exception: a somewhat disconcerting Microsoft message informing me that my fully, honestly bought-and-paid-for copy of Windows wasn’t legit.Parallels said the warning was probably triggered because Windows had been installed before its software was. In any event, I was allowed to proceed with the set-up despite my impugned integrity.Recognizing that I had already partitioned the Mac’s flash- memory storage to accommodate the two operating systems, Parallels incorporated my existing Windows programs and files into its “virtual machine” -- the imaginary PC it creates within the Mac environment. If I hadn’t already owned Windows, Parallels would have allowed me to buy and download a copy from its new online Convenience Store, which includes a variety of utilities, games and other programs as well.The software proved to be both low-overhead and low- maintenance. It took up less than a gigabyte of my constrained storage capacity and speedily launched Microsoft programs in Coherence mode from a drop-down Windows start menu accessed via a Parallels icon on the Mac’s top bar.All my custom macros for Microsoft Word worked without a hitch. I was also able to easily run the special software used for accessing the Bloomberg Professional Service, and even ran - - slowly -- Take-Two Interactive Software Inc. (TTWO)’s Major League Baseball 2K11 game.Even better, Parallels has successfully hidden many of the seams that existed in earlier versions of the program. The Mac’s camera, for instance, can now be shared between Windows and Mac programs without reconfiguring it.Many of Lion’s new features are enabled for PC programs, including gesture-based navigation, full-screen mode and the ability to pin Windows icons to the Launchpad screen. And you can even choose to replace Windows programs’ borders with a Mac look, completing the illusion of complete integration.In conjunction with the release of Parallels Desktop 7, the company also upgraded Parallels Mobile, its iPad app ($5 for a limited time, eventually $20) that allows you to remotely control your virtual machine and, now, the rest of your Mac as well. In general, I found that the app worked well but had no particular advantage over other services that offer much the same functionality.If you have a more robust Mac than my little Air, you can use Parallels Desktop 7 to manage multiple virtual machines -- not just Windows but Linux and even Lion, which could prove useful to Mac developers seeking to try out code in a controlled setting.For us normal people, though, its value comes in the meshing of the Windows applications we may need for business with the pleasures of the Mac hardware and operating system. If it doesn’t completely end the endless Windows-Mac debate, it comes pretty close: Just imagine one of those “I’m a Mac, I’m a PC” commercials with the two of them walking off arm-in-arm.