On your Virtualbox host; download the ISO of generic windows paravirtualised drivers from here:
Shut down your Windows VM and set your its Adapter Type to “virtio-net”.
Boot the Windows Vm and mount the downloaded driver ISO as a virtual CD drive.
Log on to the VM and check you can browse to the virtual CD’s \Win 7\[x86|amd64]\ folder; use devmgmt.msc to install the new drivers on the paravirtualised NIC hardware (“Trust software from Redhat”).
You shouldn’t need to to reboot.