how do I fix windows 11 26h2 installation error 0xc1900101

Use SetupDiag and the rollback logs to identify the specific incompatible driver or low-level service, then retry the 26H2 upgrade from a clean boot using installation media with Keep personal files and apps selected.

0xC1900101 is a generic rollback code, most commonly associated with driver compatibility; it does not identify which driver failed. Your available space, antivirus removal, driver updates, and SFC check already cover the usual first-line actions. Microsoft’s current 26H2 release-health page does not list 0xC1900101 as a general 26H2 known issue, although device-specific safeguard holds remain possible.^1^ ^2^

1. Identify what caused the rollback

1. Download the latest Microsoft SetupDiag.
2. Save SetupDiag.exe in a simple folder such as C:\SetupDiag.
3. Right-click Start, select Terminal (Admin), and run:

cd /d C:\SetupDiag
SetupDiag.exe
notepad SetupDiagResults.log

SetupDiag creates SetupDiagResults.log in the folder where it runs. Look near the end for a matched rule, driver filename such as xxxxx.sys, service, application, plug-in, or migration failure.^3^

If SetupDiag does not identify it, inspect these hidden folders:

C:\$Windows.~BT\Sources\Rollback
C:\$Windows.~BT\Sources\Panther
C:\Windows\Panther

Start with:

• setupact.log — the principal setup activity log.
• setuperr.log — errors reported during installation.
• setupapi.dev.log — especially useful for device-install failures.
• setupmem.dmp and .evtx files — useful if setup crashed or restarted unexpectedly.

In Notepad, search upward from the bottom for 0xC1900101, abort, rollback, abandoning, .sys, and the timestamp immediately before rollback. Microsoft specifically identifies C:\$Windows.~BT\Sources\Rollback\setupact.log for generic 0xC1900101 investigations.^4^

Once a driver or service is named:

• Obtain that component’s driver or firmware from the PC or motherboard manufacturer.
• If it belongs to software you no longer need—commonly VPN, disk encryption, virtualization, storage-management, RGB/hardware utilities, or old device software—uninstall that application rather than merely updating the visible device driver.
• Do not delete a .sys file manually. Remove its owning application or device package.

2. Retry from a clean boot

A clean boot suppresses third-party services and startup applications that can load filter drivers during setup:

1. Press Windows key, type msconfig, and open System Configuration.
2. Open Services.
3. Select Hide all Microsoft services first, then select Disable all and Apply.
4. Open Startup → Open Task Manager.
5. Disable each enabled startup item, recording what was changed.
6. Restart Windows and retry the upgrade.

Selecting Hide all Microsoft services before Disable all is critical. After testing, return to msconfig → General → Normal startup.^5^

Also disconnect nonessential external devices for this attempt, leaving only the display, keyboard, mouse, power, and—if required—network connection.

3. Upgrade without erasing your data

Back up important files first, then perform an in-place upgrade:

1. Create or download official Windows 11 installation media matching your language and edition.
2. Start your existing Windows normally. Do not boot from the USB, because that leads toward the clean-install path.
3. Open the media in File Explorer and run setup.exe.
4. Select Change what to keep.
5. Confirm Keep personal files and apps.
6. Select Install.

This option preserves personal files, installed applications, and settings; Keep nothing or booting the media for a clean installation can remove data.^6^

If the clean-boot media upgrade still rolls back, the full error including its second code—such as 0x20017, 0x30018, or 0x40017—plus SetupDiagResults.log is needed to select the phase-specific remedy. For example, 0xC1900101-0x20017 points toward a SafeOS boot failure and can involve a driver, non-Microsoft disk-encryption component, or hardware failure.^7^

References

1. Windows 11, version 26H2 known issues and notifications
2. Windows 10 upgrade error codes
3. SetupDiag
4. Windows upgrade log files
5. How to perform a clean boot in Windows
6. Reinstall Windows with the installation media | Microsoft Support
7. Upgrade fails and generates error code 0xC1900101-0x20017