sliding window over a large vector

Hi all,

I have a very large binary vector, I wish to calculate the number of 
1's  over sliding windows.

this is my very slow function

slide<-function(seq,window){
  n<-length(seq)-window
  tot<-c()
  tot[1]<-sum(seq[1:window])     for (i in 2:n) {
     tot[i]<- tot[i-1]-seq[i-1]+seq[i]
  }
  return(tot)
}

this works well for for reasonably sized vectors. Does anybody know a 
way for large vectors ( length=12 million), im trying to avoid using C.

Thanks,
Chris