That's absolutely not the case. One of Microsoft's strengths has always been backwards compatibility, and the ability to run legacy applications in compatibility mode. I work in a large organization with literally hundreds of legacy medical applications running on XP. We're planning a migration to Windows 7 as all of the apps have been certified with the exception of IE6 dependencies (which will be delivered via Citrix). We could certify approximately 0% of our applications by switching to CentOS.