for /l %i in (1,1,1) do @start /b rscript -e "system.time(for(i in
1:1e4){fisher.test(matrix(c(200,300,400,500),nrow=2))})"

   user  system elapsed
  54.50    0.04   54.63

for /l %i in (1,1,2) do @start /b rscript -e "system.time(for(i in
1:1e4){fisher.test(matrix(c(200,300,400,500),nrow=2))})"

   user  system elapsed
  55.64    0.00   55.77
   user  system elapsed
  55.92    0.02   56.08

Test of multiple concurrent R processes on Windows XP on a dual-core machine(R version 2.9.2)