NTFS: How do I find the sector(s) a file is occupying?

SysInternals' DiskView will show you which clusters a file occupies (GUi only and not files which are in use such as system files).

Run it, let it scan, then use the Hightlight row at the top to pick a file to locate. Double click somewhere on the disk-mapping to see the details.

Not sure that answers your question about sectors, but since NTFS can sit on top of hardware RAID, it can't really know which sectors it's on. Or whether it's on a disk device where sectors is a sensible thing to talk about at all (e.g. an SSD).

There is several GUI tools including SysInternals' DiskView. You can use MyFragmenter to get a text output.