Since you are searching for a string to arrive you most know what it contains. And if such string does not have a regular pattern then go back to the PC end and make the string have a pattern to it by bracketing it in a certain way. For example use the ESC character as a lead-in byte for the string (i.e. the hex code 01CH) and terminate each string with an CR character (the 00DH byte).

Now at the MCU when you start up initialize a logical variable named IN_STRING to be zero. In the serial interrupt routine have two sets of logic that steer off the value of the IN_STRING variable similar to the following concept:

If IN_STRING = 0 Then
  If RX Byte = ESC Then
    IN_STRING = 1
    BUF_INDEX = 0
  End If
Else
  STORE RX BYTE TO BUF @ BUF_INDEX
  BUF_INDEX = BUF_INDEX + 1
  If RX Byte = CR Then
    ....you have your string in buffer so 
        do what you need to do with it
    IN_STRING = 0  get ready for next string.
  End If
End If

Michael Karas