Some modems are vulnerable to an attack sometimes called a ping of death. This attack uses an escape string to access your modem. The command sent to the modem tells it to hang up. The best way to prevent this attack is by following these steps:

Step 1: right click on my computer on your desktop and select properties

Step 2: Click on the hardware tab and click on device manager

Step 3: Right click on your modem and select properties from the menu

Step 4: Click on the advanced tab and add S2=255 to the extra initialization string, now click okay

Your modem should now be secure from this attack!

 

This little fix doesn't make you invulnerable, it only protects against one attack, and you should read the Windows Security Tutorial too if you haven't already.