Sliding window over irregular intervals

Dear all,

I have some very big data files that look something like this:

id chr pos ihh1 ihh2 xpehh
rs5748748 22 15795572 0.0230222 0.0268394 -0.153413
rs5748755 22 15806401 0.0186084 0.0268672 -0.367296
rs2385785 22 15807037 0.0198204 0.0186616 0.0602451
rs1981707 22 15809384 0.0299685 0.0176768 0.527892
rs1981708 22 15809434 0.0305465 0.0187227 0.489512
rs11914222 22 15810040 0.0307183 0.0172399 0.577633
rs4819923 22 15813210 0.02707 0.0159736 0.527491
rs5994105 22 15813888 0.025202 0.0141296 0.578651
rs5748760 22 15814084 0.0242894 0.0146486 0.505691
rs2385786 22 15816846 0.0173057 0.0107816 0.473199
rs1990483 22 15817310 0.0176641 0.0130525 0.302555
rs5994110 22 15821524 0.0178411 0.0129001 0.324267
rs17733785 22 15822154 0.0201797 0.0182093 0.102746
rs7287116 22 15823131 0.0201993 0.0179028 0.12069
rs5748765 22 15825502 0.0193195 0.0176513 0.090302

I'm trying to extract the maximum and minimum xpehh (last column) values 
within a sliding window (non overlapping), of width 10000 (calculated 
relative to pos (third column)). However, as you can tell from the brief 
excerpt here, although all possible intervals will probably be covered 
by at least one data point, the number of data points will be variable 
(incidentally, if anyone knows of a way to obtain this number, that 
would be lovely), as will the spacing between them. Furthermore, values 
of chr (second column) will range from 1 to 22, and values of pos will 
be overlapping across them; I want to evaluate the window separately for 
each value of chr.

I've looked at the help and FAQ on sliding windows, but I'm a relative 
newcomer to R and cannot find a way to do what I need to do. Everything 
I've managed to unearth so far seems geared towards smoother time 
series. Any help on this problem would be vastly appreciated.

Thanks,
Irene