IE and default Mozilla override HTTP RFC and ignore file type claimed by server.
Opera strictly follows HTTP RFC and respects file type reported by server - which is misleading this time.
It is twice pity, since Opera can download from BitTorrent - if only server is configured following HTTP and MIME file types RFCs.
Known not about 4th major browser - Safari+Konqueror.
7zip is now supported in all modern archivers, from WinRar to Power Archiver.
Less technical experienced user will not download zip or 7z - it will confuse him even with ZIP built into XP - he will rather download ISO, and then crash his head thinking what to do with it.
Installer, if must be, must really be "run and forget" tool, instaling (in this context - burning) Ultimate Boot CD to system.
Otherwise, less experienced user will - turst me - open that ISO file in that very WinZIP/WinRAR/7-ZIP/ISO-Buster and "extract file to...", then he will burn that very files to usual default project in Nero Express and will crash his head tryign to boot from "ultimate unbootable CD"
He would never think ISO is somehow related to burning, he will hardly look at its icon (even will he - it would be made in same style with other archives, his multi-format archiver will support), user will just click onto it, open it into so archiver and treat it the same way as zip/7z/whatever - extract files from it to burn it to CD.
That, leaving aside what will less experienced user do with with its system, using this CD with all its tools