Last night I decided to reinstall Debian and cover the old Windows partition completely. I used the executable installer at http://goodbye-microsoft.com to install it. The installation completed, but when I attempted to install grub, it coughed up an error. Then I realized I had no bootable partition. So I went back to the partition configuration step, and the installer froze and I couldn't do anything. So I rebooted the computer and now have no bootable partition and no grub.

I do, however, have some old live CD's of Puppy Linux 4.12 and DSLinux (dunno which version). When I try to make the partition bootable in DSL, it says it can't read the drive. When I try in Puppy, I can view the files on the drive (so I know Debian is installed), but it won't let me just adjust the boot flag either.

Is it possible to adjust the boot flag and install grub via the Puppy CD?

I can also use PXE to load a bootloader, if that helps. I tried using grldr to boot it, but I'm not sure if that mounted the partition correctly. I'm also not sure if I can boot that way without a boot partition.

AFAIK you don't need the boot flag on you partition.

No GRUB is more serious. Great that you can boot in something else.

Boot
Mount the Debian disk below the /mnt directory, for example:
(Assuming you have a mnt directory and Debian is installed on sda1... you get the picture.

If Debian consists of more partitions, mount them as well in the directory tree below /mnt/sda1.

Now the trick:
and install grub with grub-install /dev/sda

While writing I realize your kernel version might not be compatible with the installed Debian version.

Recommended is to download a Debian live USB (rescue version), boot from the USB and follow the instructions above.

jlinkels

Debian uses grub2, hopefully the live CD you have uses grub2 also. But probably does not matter. This article has got me out of this bind many a times, look at section 12.