Hi Catie,
Cardinal is currently only available on the development version of Bioconductor, which requires the development version of R. If you install the development version of R from CRAN (scroll down to "other builds" on the download page), and then follow the Bioconductor installation instructions, it should work.
Cardinal will be included in the next public release of Bioconductor on April 17, at which point it will be installable from an ordinary installation of R.
Rtools includes the tools for developing R packages on Windows, and is only necessary if you are building and installing the package from source. It's not necessary if you are installing from Bioconductor.
I hope this helps.
Kyle