This brings me back two different memories. I had used this program in the past (a long while ago!) and it caused me both pain and joy.
Regarding pain: I had an MS-DOS 5.x book that spent a whole section on DEBUG.EXE, and one of the examples in it contained a command that caused the program in memory to be written to some specific sectors of the floppy disk. Guess what I tried? I executed that same command but told it to use my hard disk instead of the floppy drive. Result: a corrupted file system. Had to run scandisk (remember it?), which marked some sectors as faulty and I thought I had ruined my precious 125MB WD Caviar hard disk. It wasn't until much, much, much later that I learnt that such a thing was not possible, and that really formatting the disk with a tool that had no memory of "bad" sectors (aka, Linux's newfs) could revert the disk to a clean state. (Actually, I kept that hard disk until very recently.)
Regarding joy: On a boring weekend away from home, I used DEBUG.EXE on an old portable machine without internet connection to hack a version of PacMan. I disassembled the code until I found where it kept track of the player's lives and tweaked the counter to be infinite (or extra large, can't remember). That was fun. I could get to levels me and my father (who used to be an avid player) had never seen before!
It's a pity this tool is going, but it must go. It is way too outdated compared to current debuggers. I wonder if anyone is still using it.
Edit (Apr 1st, 2011): This is not a support forum for Windows issues. I've had to disable posting in this particular article because it was receiving lots of traffic and I don't want to moderate posts any more.