Understanding how Rescue disk works in avast 2014

HI I was wondering if I could ask some questions about Rescue Disk in avast free 2014. I was wondering so I download the ISo file to my computer and then I put it on the cd or usb drive and I get a virus say two days from now do I have to redownload the Iso file again and start with a blank cd to use it to scan my computer outside of windows?

I was also wondering when I download the Iso file and put it on a blank cd can I use the same cd say I create the Rescue disk on Windows Vista can I take that same Rescue disk and use it on Windows 7 or do I have to make separate Rescue disks for two different computers?

Thanks have a good week

You do not download any pre-made ISO. Avast creates an uptodate WinPE ISO using WAIK.