I'm currently suffering Vista to try to install some development software – on the one hand, for work, and on the other, to play with F# and dotNet.
I realised that I need the full version of Visual Studio (the free "Express" version doesn't allow any plugins, by design) so downloaded the 90-day trial version. It downloads as a .iso file to burn to disk, or to mount.
Cunningly, Vista doesn't have any builtin tools to do anything with a .iso. Now, I'm getting used to Windows not having basic functionality that you could just take for granted under Linux or Mac, but this is pretty silly for software downloaded from Microsoft itself.
A quick google for "mount iso free vista" came up with a few aborted attempts (XP only, non-free, etc.) and finally, this: Virtual CloneDrive, from SlySoft which so far seems to pleasantly Just Work.