How to boot vista from external hdd using mac!

Alright guys, so I've been going at this for hours now. I have a Macbook Pro and I'm helping some friends out and making their old vista computers brand new. I'm trying to "burn" .iso or dmg file's of vista to the hard drive and make it bootable to the hp computer. I cannot get into the hp computer though i only make it to a screen where it says where it says to restart windows to complete installation but it does it over and over, so I'm stuck. I tried already tried a DVD and was almost done installing Windows Vista but their was a blackout in my neighborhood which caused everything to stop. So now the message keeps coming up. I cant use cd;s cause files are too large....but i have two hard drive with a lot of space but need help. Any suggestions?