Don't forget to
verify the ISO files you've downloaded before attempting
to burn them.
It seems WinRAR has gotten in the act and confused things.
If you verify the "rar" files to confirm the correct MD5sum, and the md5sums are all
correct, then a simple renaming of the files from ".rar" to ".iso" should make
your burning software happy.
Are you sure they aren't ".iso" files but WinExplorer just lists them as WinRAR files?
Open Explorer go to the "Tools" menu item -> Folder Options -> View
-> and
uncheck the "Hide extensions for known file types" option
in the Files and Folders section.
If they are already ".iso" files and you didn't know it, then there's no need
to rename them.
Be sure to avail yourself of all of the helpful information in the middle
of the "Helpful Stuff" box on the lower left of the LinuxISO.org homepage.