Get a Windows 7 ISO file
Get a USB flash drive with at least 4 GB storage
Back up all files on the USB drive
Download and install Rufus
Insert the USB drive into the computer
Open Rufus
Select the USB drive under Device
Select the Windows 7 ISO under Boot selection
Set Partition scheme to MBR for BIOS or UEFI-CSM
Set File system to NTFS
Click Start
Wait for the process to finish
Safely eject the USB drive
Use the USB drive to boot the target computer
Enter BIOS or boot menu on the target computer
Set USB as the first boot device
Save changes and restart
Follow the Windows 7 installation setup