OK, I'll admit it, it was a dumb thing to do. I was using dd to copy a disk image onto an SD card for a Raspberry PI, but managed to copy the image onto /dev/sda instead of /dev/sdc. Problem now is, of course, that I've nuked the entire MBR and directory structure of the hard drive. I've reconfigured with a new drive, and this time I've dual booted the thing and taken a copy of the MBR so that if the same thing happens again, it will only clobber windows.
What I want to know is, is there any way at all to recover the file system once the first few GB of the hard drive have been overwtitten? I can use photorec to recover the files, but I think what I have is more a large collection of file fragments. I'd really like to recover the directory structure if I can.