Ever since Apple switched to Intel processors in 2006, one of the main things Mac users have wanted to do is to run Microsoft Windows alongside Mac OS X. Although Mac OS is one of the main selling points of Apple’s whole computer lineup, there are just some things you need Windows for — many games, for instance, are still not released for the Mac at all.

As of OS X 10.5, Apple has provided a piece of software called “Boot Camp” that allows you to split your Mac’s hard drive into pieces, one for Mac OS and one for Windows. This solution is known as “dual booting” — ergo, “boot” camp — and it provides the best level of performance and power to Windows. When you are in Windows via Boot Camp, your Mac becomes a normal, run-of-the-mill PC that can run any software that its hardware will support.

However, one of the major annoyances with Boot Camp is that you need to shut down and restart your computer every time you want to switch between Mac OS and Windows. This is a lengthy process that results in several minutes of downtime, making it impractical to just “hop on” to Windows for a quick game of Counter Strike or to pull up a website in Internet Explorer.

Enter the virtualization market. Several companies now offer software that allows you to run a copy of Windows inside of a “virtual machine,” all while inside of Mac OS. In fact, Microsoft itself offered such software during the Power PC days of the Macintosh, but it was slow and buggy because of the need to translate between Intel-based Windows and the PPC-based CPU. Now that all Macs have “Intel inside,” though, virtualization software has the opportunity to be pretty darn good.

However, virtual computing even in the “Mactel” era has had its fair share of problems. Hooking Windows up to fake (i.e. virtual) hardware and then translating its commands back to the native, real hardware in the Mac results in a big performance hit — if you thought Vista was slow in Boot Camp, don’t even bother trying a virtual machine. Additionally, many of the advanced hardware features that games and other applications require, such as 3D graphics, are simply unavailable to Windows while inside the virtual machine.

On Tuesday, one of the main competitors in this space, Parallels, Inc., made a move to answer many of these problems with a new version of its namesake virtualization software, Parallels Desktop for Mac. Version 4.0 of the software boasts 50 percent performance improvements across the board, and more critically, it allows far enhanced 3D performance inside the virtual machine. With full support for DirectX 9 and OpenGL 2.1, Windows-in-Parallels can now run many new, modern games that it previously couldn’t, including popular titles from the Steam library such as Half Life 2, Team Fortress 2 and Portal. One caveat here: your Mac still needs to be powerful enough to support these features. My late 2006 MacBook, for instance, has an underwhelming Intel integrated graphics chip that can’t perform well enough to allow any real enjoyment of these games. So, be sure to check that your hardware meets the game’s system requirements, or you may still run into issues.

Overall, I have found that Parallels continues to be the best virtualization offering on the Mac. Despite a hideous new icon, all other parts of their user interface are very polished. What really makes Parallels stand out, though, is the degree of integration it allows between the “guest” and “host” operating systems. In coherence mode, the Windows desktop melts away, and your applications are available right alongside all of their other Mac-native counterparts. Parallels also provides a lot of other nice touches, such as the ability to (relatively) seamlessly drag files between Mac OS and Windows, and a shared clipboard that allows you to copy something from a Mac application and paste it into a Windows application. And for Linux users out there, Parallels 4.0 is very promising. For the first time, I was actually able to install Ubuntu 8.04 without a single problem, including installation of the Parallels Tools integration software.

There are certainly other options out there — VMware’s Fusion and Sun Microsystem’s VirtualBox are probably the two most prominent alternatives — and so your own personal tastes may vary. VirtualBox is particularly interesting in that it is open-source and free to download, giving it a significant advantage over the former two products’ $80 price tags. However, both Parallels and Fusion have free trials, so you can give all three a shot and pick one you like. With all of these options available, Mac users should see the wall between them and the Windows world continue to become less defined as time goes on.