Chances are that any iso image downloaded direct to your drive will be free of any virus at least it is on your system due to antivirus scanners employed on servers themselves. Most pc viruses are spread by one or more infected machine where contact addresses taken from the address book in Outlook Express, Outlook, Mozilla'sThunderbird(not anywhere near as likely there), or another type of contact list(msn messanger?) then spreads that to other machines which further spread and so forth. If your drive has a bug and infects the downloaded iso image then the burn to disk will contain it there too. Having good antivirus protection inplace will often prevent this unless the iso image is downloaded from... a questionable source? Legitimate sites like those for Linux and game manufacturers will employ antivirus as well as firewalls to protect their servers from contamination.