Excel + R = BERT
The Basic Excel R Toolkit (BERT) is an Excel add-in designed to run R functions in Excel spreadsheet cells. BERT makes it easy to add your R functions and call them in Excel (it's hard to imagine a way to make it any easier, but if you have any thoughts please let us know).
Just to be clear, BERT uses its own completely isolated R environment. It won't be affected by (or screw up) any other R installation on your machine. And that R environment is set up automatically by the installer, so it's basically one step.
BERT supports Excel 2007, 2010 and 2013 in both 32- and 64-bit flavors.