Published June 28, 2026 - Updated June 28, 2026 - By Xilly

How to Dual Boot Windows Without a USB Drive

Install a completely separate Windows as a dual boot without touching your current setup and without a USB drive. Just an ISO, a script, and a partition.

PC Optimization Guides

Windows Guide · Dual Boot

By XillyPlatform: Windows 10 / 11No USB Required

A clean dual boot Windows install without a USB stick, DVD, or media creation tool. Just your PC, a Windows ISO downloaded from Microsoft, and a script that does almost everything automatically. Your current Windows stays completely untouched.

What You Need

Windows ISO

From Microsoft

Download the ISO version (not the media creation tool) from microsoft.com. Choose 64-bit.

Free Space

50 GB minimum

On the drive you plan to shrink. If you do not have this, use a different drive or clear space first.

The Script

From Discord

Join the Xilly Discord and download from the tweak files channel. Comes as a zip with installwindows.bat inside.

Step 1: Create a New Partition

Press Windows key + R, type diskmgmt.msc, and open Disk Management. Right-click your main drive, click Shrink Volume, and enter how much space to shrink in megabytes. For 100 GB, type 100000.

Once the unallocated space appears, right-click it, click New Simple Volume, press Next through the wizard, name the partition (example: Second Windows), and finish. Note the drive letter assigned. For this example it is drive H.

Remember the Drive Letter

You will need to type this letter when the script asks. If the partition was created without a drive letter, open Disk Management, right-click the partition, and use Change Drive Letter and Paths to add one.

Step 2: Run the Script

1
Extract the zip
Extract the downloaded zip file to somewhere easy to find like your desktop or C drive. You will see installwindows.bat inside.
2
Run as Administrator
Right-click installwindows.bat and click Run as administrator. The script will confirm you have created a new partition before continuing.
3
Select your ISO
A file picker will open. Navigate to the Windows ISO you downloaded from Microsoft and double-click it.
4
Enter the drive letter
Type the drive letter of the partition you created (example: H) and press Enter. Confirm when asked. The script will apply the Windows image to that partition.

When complete, the script automatically sets the boot entry so your PC knows two Windows installs exist, then cleans up the temporary files. You will see: Installation complete, Windows has been installed to drive H, reboot your PC and you will see a boot menu.

Step 3: Boot Into Your New Windows

Restart your PC. A boot menu will appear. The top entry is the newly installed Windows. Press Enter and it will set up like a brand new PC on that separate partition. Your original Windows is completely untouched.

From here, go through the normal Windows setup: region, language, and account. Once done, you have two completely separate Windows installs on the same drive. Every time you boot, you pick which one to use.

Troubleshooting Common Errors

Built-In Error Messages

Not enough space on C

Free up space

The script needs at least 8 GB free on C for extraction. Free space and try again.

Drive letter does not exist

Check Disk Management

Make sure the partition has a drive letter assigned. Use Change Drive Letter and Paths in Disk Management if needed.

DISM failed

Check space and ISO

Most common causes are a partition that is too small or a corrupted ISO. Re-download the ISO if the problem continues.

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About the Author

Xilly works on real gaming PCs with manual BIOS, Windows, driver, and game-specific optimization for competitive players. Learn more about Xilly and our optimization process.

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