I just acquired a old box without a hard drive, although it has a product key certificate for Windows Vista on it. I know it is possible to download ISOs for Windows 8, but is it possible to get one for Windows Vista, or do I have to track down a copy of Windows 7?
If the old box is manufactured by DELL or any other large manufacturer, you can get hard copies of the software installed by contacting them and quoting the serial number. It will cost but not a great amount.
Windows Vista can be downloaded from Digital River, not as ISO though, but as a WIM image, which you can convert to ISO using Windows Vista WAIK. You can find details here: http://www.heidoc.net/joomla/technology-science/microsoft/57-windows-vista-direct-download-links