When I run the line on my laptop,
table500 <- readHTMLTable('http://en.wikipedia.org/wiki/List_of_S%26P_500_companies')[[1]]
it gets the data. When I run it on my desktop, I receive the error
Error: failed to load external entity "http://en.wikipedia.org/wiki/List_of_S%26P_500_companies".
So I'm guessing the problem has something to do with network settings in my desktop, I haven't the slightest idea what it could be though. Any suggestions?