A common piece of advice runs through recent conversations with companies migrating to Windows 7: test early and often and don't be afraid of virtualization.Windows 7 adoption has certainly been on the rise, easily outpacing its predecessor, Vista. After seven months on the market, Windows 7 owns 13 percent of the global OS market share, according to Web metrics company Net Applications. Adoption rates have been lifted by corporate PC refresh cycles and a recent jump in PC sales. In May, Gartner predicted a 22 percent growth of global PC sales for 2010. As for corporate adoption, a recent poll of 923 IT pros conducted by Dimensional Research found that 16 percent are already running some Windows 7 and 42 percent plan to start deployment by the end of this year.Also boosting Windows 7 corporate adoption rates: Many businesses had time to prepare and test for application compatibility because they simply passed on Vista. And new application and desktop virtualization tools from Microsoft, VMware, Citrix and others have made deployment easier.Two early enterprise adopters of Windows 7 recently shared deployment lessons learned with CIO.com.Using Virtualization for Migration, Intallation and Remote DesktopsBoth Expedia and Continental Airlines are in the throes of Windows 7 migrations, and both have used to virtualization technologies to get there.Travel site Expedia is using both application and desktop virtualization tools as it migrates from Windows XP to Windows 7. After a year of planning that included three phases (application compatibility testing first, a pilot testing program next and finally a wider roll out of Windows 7 to new employees and those with old PCs) Expedia now has Windows 7 deployed to 300 seats, with plans to roll it out to 2,500 seats by the end of the year.[ For complete coverage on Microsoft's new Windows 7 operating system -- including hands-on reviews, video tutorials and advice on enterprise rollouts -- see CIO.com's Windows 7 Bible. ]The travel site used application virtualization as a cost-saver to package and install applications using Microsoft's App-V virtualization tool (part of the MDOP suite). Previously, Expedia's application deployment method had been more or less manual, but the cost of packaging each of the apps was unsustainable, says Mike Peterson, Expedia's manager of end-user technology."We looked at the cost of just moving to a virtualized packaged or a virtualized instance of that application," says Peterson, "and we went from 24 to 30 hours worth of work to four to eight hours, a huge time and cost reduction."Expedia is also looking at desktop virtualization through Microsoft's VDI (Virtual Desktop Infrastructure) tool. The company will mainly use this technology for its workers in contact centers that are using thin clients. With VDI, the users can access full desktop environments that are really residing in virtual machines running on servers in the data center.Continental Airlines, which has deployed Windows 7 to more than 2,000 seats to date, is also using about "180 Windows 7 VDI sets" for its "super agents", who take reservations and do back office work. "You have all these applications and you just want to know that they work; you don't want to have to reverse engineer them," says Eric Craig, director of network and systems engineering at Continental. "And our experience is that if you throw them on a VDI asset, they work."Continental currently provides its super agents with virtual Windows 7 desktops that allow them to work from home and access everything as if they were running Windows 7 locally. To do this the airline is using a Terminal Services Gateway, a feature of Windows Server 2008 that allows remote users to connect to physical or virtual applications or desktops on a corporate network, from any Internet-connected machine.The Importance of Application Compatibility TestingIn preparing for its Windows 7 migration, Continental started testing Windows 7 in early 2009 in the middle of Vista deployment, and liked what it saw enough to halt the move to Vista even after many employees had Vista running on its PCs. A lot of the testing the airline did to go from XP to Vista paved the way for the move to Windows 7, Craig says. "We did a lot of app compatability testing when we made the leap to Vista," says Craig. "We had to rewrite some applications. We had to go to our vendors. So I already know those Vista-ready applications are going to work just fine on Windows 7."Expedia never upgraded from Windows XP to Vista and is earlier in its Windows 7 migration than Continental. But Expedia's Peterson says the company benefited from its systematic three-phase approach to deployment and extensive application testing.\nSlideshow: Seven Features in Windows 7 You Probably Don't Know About\nSlideshow: Windows 7 in Pictures: The Coolest New Hardware\nSlideshow: Seven Tools to Ease Your Windows 7 Rollout\n\nAfter an initial scan of 22,000 individual applications, Expedia slimmed it down to 450 apps that run as business critical, of which the company determined that 200 were installed to more than 50 people.Expedia focused on those 200 apps to test for compatibility with Windows 7, to see which ones would need to be virtualized in some way."There were about 20 apps that we determined will not be Windows 7 supported with their current release version that we use in our environment," says Peterson.Expedia then resolved that seven of those apps were never going to be supported on Windows 7, which could mean that the company doesn't exist anymore. So Expedia needed to figure out whether to move to a different product or invest in an upgrade of the application. In the end, because of all of Expedia's methodical testing, app compatiblity with Windows 7 was not an issue."Fortunately for my team those seven final applications are all end of life-ing in the company by the end of this year, so we had no application problems," says Peterson. "It was a huge win for the team, because having to overcome application hurdles can be very time consuming and costly."Shane O'Neill is a senior writer at CIO.com. Follow him on Twitter at twitter.com\/smoneill. Follow everything from CIO.com on Twitter at twitter.com\/CIOonline.