If you must download a pre-built QCOW2 file (often found on virtualization marketplaces or OSboxes), proceed with extreme caution.
qemu-img check -r all windows7.qcow2
First, increase the QCOW2 size (e.g., +10 GB): windows 7 qcow2 file
This will create a new qcow2 file called windows7.qcow2 with a size of 50GB. If you must download a pre-built QCOW2 file