Build ISOs including VirtIO drivers

Prep Machine

Launch a fast Windows 7 box and create a folder c:\tmp
install AIK for Windows 7 (http://www.microsoft.com/en-us/download/details.aspx?id=5753)
install an ISO editor such as UltraIso
unpack inject-drivers.zip to c:\tmp

copy all your Windows ISOs to folders c:\tmp\win7, c:\tmp\win2008, c:\tmp\win8, c:\tmp\win2012, etc
extract sources\boot.wim and sources\install.wim from each iso and put them in the same folder as the iso
get the lastest virtio-win-x.x-xx.iso and extract the drivers into the VirtIO folder for each OS (e.g. WIN8\AMD64\*.* from the iso goes into c:\tmp\win8\VirtIO\ and into c:\tmp\win2012\VirtIO\)

Start All Programs -> Microsoft Windows AIK -> Deployment Tools Command Prompt (Run as Administrator)
check how many indexes your install.wim has (e.g. dism /get-wiminfo /wimfile:c:\tmp\win2008\install.wim). Windows 2008 has up to 8 indexes
Set the variable IDX in each winxxxx-inject.bat to the number of indices you found in each install.wim (e.g. IDX=8)
For each OS there is a SetupComplete.cmd. This script is always executed by the Windows installation as a final step. Make sure it contains the ballon service installer. You may add other commands to it.

launch multiple winxxxx-inject.bat sequentially and uninterrupted, e.g.: winpe-inject.bat && win7-inject.bat && win2008-inject.bat && win8-inject.bat && win2012-inject.bat
alternatively you can run multiple batch commands in parallel by launching multiple instances of "Deployment Tools Command Prompt" as Administrator
for each iso use UltraISO to copy boot.wim and install.wim back to the sources folder of the ISO file

Use virtio dirvers for best performance. More info: Paravirtualized Block Drivers for Windows