Windows 7's XP Mode
To use XP Mode (shown at right), your PCs must support processor based virtualization (either Intel-VT or AMD-V), which leaves out a lot of older PCs. Users start and run applications in XP Mode just as they do native applications; however, these applications cannot take advantage of native Windows 7 features such as the Aero interface.
From the software perspective, you may want to look into the Application Compatibility Toolkit (ACT) 5.5; it has a variety of tools that help determine which applications will function smoothly in Windows 7 and which ones will give you trouble. ACT does require a local agent be installed on the systems you are testing; those agents drill down into the systems to find every last application you've forgotten about and report them back to the server. Typically you don't need to scan every single machine in the enterprise; a few representative machines from different departments and locations should provide enough of a baseline.
The Microsoft Standard User Analyzer (SUA)
One of my personal favorite tools from the Application Compatibility Toolkit is the Standard User Analyzer (SUA), shown at right, which requires you to install the Application Verifier tool. SUA lets you loosen User Access Control (UAC) permissions for standard users for those applications that UAC causes to issue many "are you sure?" messages. (UAC's overeagerness was one of users' main dislikes of Vista.)
Addressing the licensing question
Now that you've done the footwork and have PCs ready to deploy, it's time to think about licensing and activation. In enterprise deployments, it's best to use a volume license and the product activation approach called Volume Activation (VA) introduced with Vista. To use VA, you need either a Multiple Activation Key (MAK) or a Key Management Server (KMS), which requires a KMS key. The mechanics of getting these keys is complicated, so Microsoft has a help page on how to do it.
Note that Microsoft has many types of licenses for Windows and other products. When upgrading to Windows 7, be sure not to confuse the volume license with the Enterprise Agreement, Enterprise Subscription Agreement, or Software Assurance (SA) programs. The two agreements are essentially maintenance agreements that include the SA program, which lets you upgrade to a new version of Windows at any time during your agreement period, in exchange for a per-system annual fee. Given Microsoft's slow OS update schedule and the lack of transferability of SA coverage to new PCs, this insurance-type plan has ended up costing businesses more per desktop than simply purchasing upgrade licenses for old PCs and getting the OS included with new system purchases.