what is 16 bit MS DOS Subsystem

The 16 bit subsystem is what allows some older MS-DOS software to run in today's Windows NT and XP. It sets up a simulated MS-DOS environment, and many old applications can run fairly well. (This not to be confused with the Command Shell, which is a true 32 bit Windows application.)

But in this case it looks like some old 16 bit MS-DOS application is attempting to start when you log in. In fact, it's possible that the 16 bit debugging program debug.exe is being started.

If you had just installed some new software, I might suspect that as the cause. Especially if it was a new install of some old software package. However my assumption is that this isn't the case.