dplyr makes this easy:
ID <- c(1,1,1,1,1,2,2,2,2,2,3,3,3,3,3)
p1 <- c(21000, 23400, 26800, 2345, 23464, 34563, 456433, 56543, 34543,3524, 353, 3432, 4542, 6343, 4534 )
p2 <- c(234235, 2342342, 32, 23432, 23423, 2342342, 34, 2343, 23434, 23434, 34, 234, 2343, 34, 5)
my.df <- data.frame(ID, p1, p2)
library(dplyr)
df_scaled <- my.df %>% group_by(ID) %>% mutate(p1 = scale(p1), p2=scale(p2))
Note that there is a bug in the stable version of dplyr when working with scale; you might need to update to the dev version (see comments).
