[r] Adding simple legend to plot in R

I would like to create a very simple plot. I am using this data:

a <- read.table("https://dl.dropbox.com/u/22681355/a.csv", sep=',', header=TRUE)

plot(a$X25, type="l",col="red", ylim=c(0,100))
lines(a$X25.1, type="l", col="blue")
lines(a$X25.2, type="l", col="green")
lines(a$X25.3, type="l", col="brown")

Now I would like to add a simple legend that indicates which color is which variable.

I understand that I can use the legend() command, but my problem is that I don't know how to put colors next to the text in the legend.

What's the simplest command that would do this?