Keep a clean, pristine copy of your windows7.qcow2 file designated as a read-only backing file. Create thin external snapshots for day-to-day use. If the operating system becomes infected or corrupted, simply delete the snapshot and start fresh.
You can save the exact state of the OS and revert to it instantly. Windows 7.qcow2
# Create a 40 GB dynamically allocated QCOW2 image qemu-img create -f qcow2 Windows\ 7.qcow2 40G Keep a clean, pristine copy of your windows7