Names of variables and functions in R are case-sensitive and maybe comprised of any number of alphabetical characters (a-z, A-Z), numerics (0-9) and period (.). You can't use a number in the lead position of a name.
If the operating system is MS Windows then you must take care when referencing pathnames to include "\\" rather than just "\". For example, to read in a data table you might enter a command such as the following:
read.table("c:\\survivaldata\\leuk.dat", header=T)