diff --git a/README.md b/README.md
index 3607d63..5109e05 100644
--- a/README.md
+++ b/README.md
@@ -46,8 +46,10 @@ snap connect qemu-virgil:removable-media
 
   * Download a .iso image of a Linux distribution
   * Create a VM configuration file; for example `ubuntu.conf`
+    * The **default** `guest_os` is `linux`, so this is optional for Linux VM configs.
 
 ```
+guest_os="linux"
 iso="/media/$USER/Quickemu/ubuntu/focal-desktop-amd64.iso"
 disk_img="/media/$USER/Quickemu/ubuntu/focal-desktop-amd64.qcow2"
 disk=128G
@@ -64,6 +66,7 @@ Which will output something like this:
 ```
 Starting /media/martin/Quickemu/ubuntu-focal-desktop.conf
  - QEMU:     /snap/bin/qemu-virgil v4.2.0
+ - Guest:    Linux optimised.
  - BIOS:     Legacy
  - Disk:     /media/martin/Quickemu/ubuntu/focal-desktop-amd64.qcow2 (64G)
  - ISO:      /media/martin/Quickemu/ubuntu/focal-desktop-amd64.iso
@@ -87,13 +90,13 @@ You can use `quickemu` to run a Windows 10 virtual machine.
   * [Download Windows 10](https://www.microsoft.com/en-gb/software-download/windows10ISO)
   * [Download VirtIO drivers for Windows](https://docs.fedoraproject.org/en-US/quick-docs/creating-windows-virtual-machines-using-virtio-drivers/index.html#virtio-win-direct-downloads)
   * Create a VM configuration file; for example `windows10.conf`
-    * The `VIDEO_DRV=qxl` line instructs `quickemu` to use a Windows compatible video driver.
+    * The `guest_os="windows"` line instructs `quickemu` to use optimise for Windows.
 
 ```
+guest_os="windows"
 iso="/media/$USER/Quickemu/windows10/Win10_1909_English_x64.iso"
 driver_iso="/media/$USER/Quickemu/windows10/virtio-win-0.1.173.iso"
 disk_img="/media/$USER/Quickemu/windows10/windows10.qcow2"
-VIDEO_DRV="qxl"
 ```
 
   * Use `quickemu` to start the virtual machine:
@@ -107,6 +110,7 @@ Which will output something like this:
 ```
 Starting /media/martin/Quickemu/windows10.conf
  - QEMU:     /snap/bin/qemu-virgil v4.2.0
+ - Guest:    Windows optimised.
  - BIOS:     Legacy
  - Disk:     /media/martin/Quickemu/windows10/windows10.qcow2 (64G)
              Just created, booting from /media/martin/Quickemu/windows10/Win10_1909_English_x64.iso
diff --git a/quickemu b/quickemu
index ddd92e8..7ecd51f 100755
--- a/quickemu
+++ b/quickemu
@@ -113,6 +113,33 @@ function vm_boot() {
     echo " - BIOS:     Legacy"
   fi
 
+  # If guest_os is unset, default to "linux"
+  if [ -z "${guest_os}" ]; then
+    guest_os="linux"
+  else
+    # Force to lowercase.
+    guest_os=$(echo ${guest_os,,})
+  fi
+
+  # Make any OS specific adjustments
+  case ${guest_os} in
+    linux)
+      VIDEO_DRV="virtio-vga"
+      ;;
+    windows)
+      VIDEO_DRV="qxl"
+      ;;
+    *)
+      echo "ERROR! Unrecognised guest OS: ${guest_os}"
+      exit
+      ;;
+  esac
+  echo " - Guest:    ${guest_os^} optimised."
+
+  if [ -z "${disk}" ]; then
+    disk="64G"
+  fi
+
   echo " - Disk:     ${disk_img} (${disk})"
   if [ ! -f "${disk_img}" ]; then
       # If there is no disk image, create a new image.