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Nobody at Microsoft was prepared for the sheer level of devotion that users and organizations alike would show for Windows XP after its release. Here we are, 12 years after its debut, and Windows XP is still commanding 25% or so of the operating system market share. Granted, Windows 7 is in first place with a healthy 50% or more, but the mere facts that Windows XP still has such a devoted following and that its market share has dropped by only about 1% a month means many stragglers will still be migrating from Windows XP to Windows 7 in the months and years to come.
Migrating from Windows XP to Windows 7 isn't a single-step process for a variety of reasons, most of them due to the architectural differences between the two OSes. Rather than provide a direct upgrade path, Microsoft created ways to allow user profiles and data to be migrated from an XP installation to a Windows 7 one. That still leaves open a great many questions about application support and other functions. Not all of these questions have easy answers, but I'll attempt to address the major issues that come up when migrating from Windows XP to Windows 7, either on a single machine or in a whole site.
The OS itself
The first and hardest truth about migrations from Windows XP to Windows 7 is that there is no direct upgrade path. Despite the sheer number of people who skipped Vista entirely and make the leap from XP to 7 -- many of them corporate customers -- Microsoft doesn't have an easy way to make this happen.
For better or for worse, Microsoft has concentrated its efforts on making the future generations of its operating system, such as Windows 8. It's easy for latecomers to assume that the Windows upgrade path has been thoroughly paved in the intervening years. Sadly, it hasn't been.
Some folks have pointed out it's theoretically possible to upgrade from Windows XP to Windows Vista and then upgrade from Vista to Windows 7. One attempt, for example, tried it and found the whole migration process to be more hassle than it was worth.
This is further complicated by the fact that you need valid installation media (and keys) for Windows Vista and 7, so the whole thing is best considered an academic exercise, not a practical solution.
This brings us to the first and most important thing about migrating from Windows XP: The OS itself should be the least of your concerns. Think instead about applications, user data, system compatibility and so on. That way, you won't be distracted by a false goal and expend too much energy on it.
Because there's no direct migration path to Windows 7, assume that every app must be reinstalled from scratch. Any applications for which an installer cannot be found will need to either be replaced entirely or migrated by hand to the new environment.
One discussion thread talks about some of the difficulties involved in manual migrations. It mentions a few tools -- FileMon and RegMon, since superseded by Process Monitor -- that could be used to capture the program's behaviors as a guide to reproducing them in a new environment.
Most of the applications that run on Windows XP also run on Windows 7 as is, although there are some exceptions. Most notable are 16-bit apps, which require the 32-bit version of Windows 7 to run properly. The 64-bit edition of Windows 7 does not have the needed 16-bit compatibility layer.
Workarounds include using Windows 7 XP Mode or installing a 32-bit copy of Windows 7 in a virtual machine running on a 64-bit edition of Windows. The ultimate solution, of course, is to leave behind as many of your 16-bit applications as possible. It's likely that any 16-bit apps that are still running are custom creations that can't be abandoned wholesale, only rewritten from scratch. If that hasn't already started happening, it ought to be.
Migrating user data
Because migrating user data also typically involves copying application settings and user data, it's impossible to talk about apps without some discussion of user data.
More about migrating from Windows XP to Windows 7
It's not too late to plan for a Windows 7 migration
Preparing to upgrade from Windows XP to Windows 7
Tools for migrating Windows XP apps to Windows 7
Reasons to avoid using an external hard drive to install Windows 7
Comparing editions of Windows XP and Windows 7
In some ways, migrating user data is the easiest part because Microsoft provides a few different ways to move data automatically between Windows XP and 7 installations. For individual users, the Windows Easy Transfer tool can copy user data directly to another machine or to an archive file, which can be stored on a network or portable drive.
For mass migrations, the User State Migration Tool is a better deal, since it can be scripted and used to perform large-scale migrations. The latter has a number of graphical front ends (e.g., USMT GUI or M.U.S.T.) to make easier the job of customizing the migration process.
The less dependency there is on user data on a given machine, the easier upgrading enterprise desktops will be. If user data is stored primarily on network shares or synced via Active Directory, this spares you most of the heavy lifting during the migration process itself. This rule applies for almost any OS-to-OS migration, but it is no less relevant with Windows XP.
The radical differences between Windows versions and the sheer number of potential migration problems may require IT administrators to designate a window of time during which the results of the migration should be followed closely.
The first thing to keep in mind is user future shock. It's easy to forget the vocal minority of people who resented Windows 7's changes. Many of those users were -- you guessed it -- longtime Windows XP users who didn't like being dislodged from their existing work habits.
The more vociferous users are about the change, the more likely they'll benefit from retraining (as with the Windows 8 interface), freedom of customization (turning off Aero Glass or other visual elements specific to Windows 7, for instance) or other gestures designed to let them preserve their productivity.
Long-term application behaviors also need to be kept in mind. Just because something seems to work once it has migrated doesn't mean it'll work properly in all cases, so a conduct thorough shakedown over weeks or months.
The full gamut of an application's behavior isn't always immediately apparent. This goes double for software that is tightly coupled to hardware, such as software suites for multifunction devices where you might not use all of the device's functions at first, and where failure of some kind may be in store further down the line without you knowing.
The time to start migrating away from Windows XP passed a while ago, not just because of Microsoft's own efforts to move to Windows XP end of support. The rest of the software and hardware world had started doing the same some time ago as well.
The whole Windows ecosystem has become much less friendly to Windows XP. While time may seem to stand still in certain environments such as those where custom apps or other bits of "frozen infrastructure" take root, Windows upgrade requirements can catchup to an organization. Don't fall too far behind if you can help it.