Since this anomaly doesn't occur when using Hyper Terminal I would guess that there's nothing wrong with the USB/Serial drivers. While your code is rather odd and using 'End' is not good programming practice, there is nothing in your code that would cause a system restart. It's more likely that something went wrong when VB was installed.

Here's an experiment for you. Find the port that your internal modem is assigned to. It's usually COM3. Then try to open it with MSComm. This will eliminate any question of the USB/Serial converter.

BTW, I have four (and still growing) USB/Serial converters and they all work seamlessly, as if I had real serial ports. Their primary use is with MSComm and Microcontrollers etc.
Code:
Option Explicit

Private Sub Command1_Click()
   MSComm1.PortOpen = True
   MSComm1.Output = "ATV1Q0" & Chr(13)
End Sub

Private Sub Form_Load()
   MSComm1.CommPort = 3
   MSComm1.RThreshold = 1
End Sub

Private Sub MSComm1_OnComm()
   If MSComm1.CommEvent = comEvReceive Then
      MsgBox "The Modem responded with " & MSComm1.Input
      MSComm1.PortOpen = False
   End If
End Sub