search for: arrrang

Dear all, I am a newbie in R. I encounter a problem as follows. I have 2 vectors X and Y that have a equal length of several thousand. I see Y as the function of X. Both of them are random. X is not arrranged in any order. Of course, I do plot(X,Y). Now, I want to use a sliding narrow window to run over each X, then calculate the variances within that window. Anyone knows easy way in R to do this? Reply is appreciated. -MY