Automation may not be needed in a small company, which can upgrade a handful of computers manually. Since Microsoft does not provide an in-place upgrade from XP to Windows 7/8, the process involves storing user data and settings on external media and then restoring after the new OS is up and running. It's not particularly difficult, just an extra step.
However, for a large company with hundreds or thousands of computers to migrate, automation is critical. This will typically involve creating a standard OS image, which includes the OS plus applications that need to be installed. This is accompanied by a set of logic for installing the OS correctly on different computers. The unattended installation can be handled by a variety of technologies, with Microsoft System Center Configuration Manager (SCCM) being the best known.
Bandwidth is an easy thing to overlook for operating systems deployment, but it's a huge factor for companies with offices all over the world. Merely applying a service pack in large organizations is a serious and complex undertaking. For example, the Windows 7 Service Pack 1 involves the transmission and processing of one gigabyte of information for each computer to be upgraded.
If a company has 20,000 computers and needs to send the service pack to each, that's 20 terabytes of traffic. Of course, files aren't moved over the WAN to each PC, they are moved to a local cache of some kind at each operating location's WAN, and served to PCs from there. OS image files can be up to 20GB or more, and they require regular updating when applications, drivers, etc., are updated. So, even just populating or repopulating this image is a WAN bandwidth challenge.
Infrastructure isn't something people usually consider when they think of migrating computer operating systems. But beside WAN bandwidth and possibly technology to efficiently use that bandwidth, Preboot Execution Environment (PXE) points are required to install new operating systems on computers. These have to be placed on each network segment, unless you're willing to change router configurations (IP helpers, DHCP scope options).
This is also true with state migration points (SMP), the storage space to save users' data and settings before overwriting XP and to apply after installing Windows 7/8. SMPs need to be on the same LAN as the PC being updated to prevent traffic flooding the WAN. Some technologies such as peer-to-peer PXE and virtual SMP can prevent the need to deploy physical servers at sites, by enabling these functions on local clients without perceptibly impacting their performance.
Get it done
If you work in an enterprise that has not yet begun the OS migration process, here is a high-level overview of what needs to happen:
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