Some IT admins are complaining that Microsoft is slowing networks to a crawl by unexpectedly pushing Windows 10 upgrade files to networked PCs.

Firms are complaining that Microsoft is slowing their networks by pushing Windows 10 files to computers they don't want to upgrade.

Over the past couple of days various IT admins in the US and the UK have reported slow network speeds as Windows 10 upgrade files were unexpectedly downloaded to Windows 7 and 8.1 machines.

Windows Update appears to be delivering the files to centrally-managed networked PCs on a Windows Domain. Affected machines appear to not be being managed using Windows Server Update Services (WSUS).

IT pros managing these networks had been under the impression that Domain-joined PCs would not receive any downloads without the upgrade being initiated by an administrator - based on guidance issued before last week's Windows 10 launch.

But Microsoft appears to be pushing to these machines files that would be needed should the businesses decide to upgrade to Windows 10. Between 2GB and 3GB of files are reportedly being downloaded to each machine and stored in a hidden folder on the Local Disk called $Windows.~BT.

One sysadmin who looks after a network of about 250 machines said the downloads were, "Basically using all the bandwidth on our network. We usually get around 50mbps down and now we are at around 5mbps. As soon as a few finish 10 to 11 more computers start going," adding, "I can't believe Microsoft is pushing this automatically".

The downloads appear to be taking place on machines without the white 'Get Windows 10' logo on the Taskbar, further confusing admins as to why these machines should be being prepared for upgrade.

"None of the users we have interviewed recalls seeing or clicking any Windows 10 reservation or installation," said another IT manager. He said downloads had taken place on about half the 150 machines he managed, causing, "Significant network saturation on anything under 10M fiber".

For those wanting to block the upgrade files from being downloaded, it seems Microsoft recently updated its Group Policy settings in Windows Server 2012 R2 to allow the upgrade to be turned off on all Windows 7 and 8.1 PCs on a Domain.

The setting is accessible via [Computer Configuration > Policies > Administrative Templates > Windows Components > Windows Update] and can be seen below.

The addition of the setting again surprised the admin who uncovered it, as he also "didn't think the 'Get Windows 10' app was supposed to show up on a Domain-joined PC".

When asked to comment on why these downloads were happening and whether it was the users or the update process that was at fault, Microsoft would only respond with a high-level statement.

"For individuals who have chosen to receive automatic updates through Windows Update, we help upgradable devices get ready for Windows 10 by downloading the files they'll need if they decide to upgrade. When the upgrade is ready, the customer will be prompted to install Windows 10 on the device," a Microsoft spokesman said.

"For organizations, IT professionals have access to tools, which enable them to manage and control downloads on their network domain, and also make a determination as to when and if to install Windows 10.

"This approach is an industry practice, which stages software, reducing the time for installation and ensures the device is ready (e.g., has the correct graphics drivers to ensure display monitor will function correctly), should the customer choose to install the upgrade."

What's unclear is whether this will be a one-off download or whether Domain-joined PCs will continue downloading Windows 10-related files as Microsoft update the new OS. For example, a large security update for the operating system was released yesterday.