reorder columns based on values in a particular row.

x <- structure(list(aa = c(3L, 5L, 7L, 33L), bb = c(4L, 4L, 8L, 63L),
  cc = c(5L, 3L, 6L, 55L)), .Names = c("aa", "bb", "cc"),
  class = "data.frame", row.names = c("1", "2", "3", "100"))
x[,order(-x[nrow(x),])]

Building on Joshua Ulrich's answer, in case you want to sort by the row name, rather than number:

x[, order(-x[which(rownames(x) == '100'), ]) ]

where 100 is the row name, as in the example above.