Answer:
The jack for an analog phone line (the "regular" kind, where you could plug in a $10 phone and hear a dial tone) looks EXACTLY the same as the jack for a digital phone line, such as would be used in a business. They LOOK identical, but the line voltage and the signaling are completely different. Phones, modems, etc. that are designed to work with analog lines will not work, and can be damaged, if they're plugged into digital lines.
Digital Line Detect (DLG.EXE) is a program that's included with the modem driver software in most Windows notebooks (and some other computers.) It runs in the background, making sure you don't plug your modem into a digital line. If you do, it shuts down the modem to prevent damage and pops up a warning message.
If you're sure you won't be making that mistake, you can prevent Digital Line Detect from starting every time Windows boots up. It won't hurt anything.
Also - DLG.EXE is sometimes flagged as spyware, because the company that produces it also produced some spyware in the past. DLG.EXE itself is safe - but, as I said, you don't need it if you don't plug your modem into unfamiliar jacks.