Even an old legacy BIOS is no problem.
Yesterday I installed Win11 with following components:
CPU: Intel Core 2 Duo E8500 3.16GHz overclocked to 4GHz
Mainboard: Gigabyte EP35-DS3 a really really old motherboard
This time you have to go the other way round because the old legaycy BIOS cannot handle the GPT filesystem the Windows 11 installation stick is using.
You get a message that this stick is only bootable from an UEFI-BIOS.
You cannot create an installation stick using the media creation tool because this formats your stick with FAT32 and the installation file for Win11 bigger than 4GB.
So you have to format your usb drive with NTFS.
Download the Win10 iso file with media creation tool.
Open the Win10 iso file with a tool like "AnyBurn" and remove from folder "sources" the files "appraiserres.dll" and "install.esd".
Open the Win11 iso file and take the files "appraiserres.dll" and "install.wim".
Rename "install.wim" to "install.esd".
Add "appraiserres.dll" and "install.wim" to the folder "sources" in the Win10 iso file with AnyBurn.
Save the iso file.
Create a bootable usb stick with this hybrid Windows installation iso file.
First time I tried to install it on this old system I thought it wouldn't work.
Windows logo popped up very shortly and then the only thing you could see was a blinking cursor in the left upper corner of the screen.
So I aborted.
Next I checked with an old system with biostar H61MGV3.
Windows logo - blinking cursor and after about 20sec the installation screen was provided.
Next system with an old P5 board - blinking cursor and after about 1min installation screen.
So I got back to the GA-EP35-DS3 system and after some 5-10min
the screen changed to blue and now you had to wait for next screen everytime a few minutes.
I thought at this stage that it was a really really bad thing to try to install Win11 on such an old system but I changed my mind back.
Win11 is booting and shutting down much faster than the old Win10 before.
Every single driver has been implemented by Win11 itself.
Everything is working - PT of course too.