[R] How to use as.Date (or something else) with "31-Jul-2010 23:59:00"

[Eduardo M. A. M.Mendes]

Hello

I have a huge file (not an R-file) in which the first column is a string
with date, hour, minutes and seconds (For instance, "31-Jul-2010 23:59:00").

I tried as.Date but the error msg was "Error in charToDate(x) : 
  character string is not in a standard unambiguous format".   

I have checked the help for the function as well as date but to no avail.

How can that sort of string be transformed to something useful in R?

Many thanks

Ed