There are several threads regarding incompatibility of libraries that make novel use of interrupts. Here's one:
http://arduino.cc/forum/index.php/topic,126251.0.html
NewPing is a common culprit, as is NewSoftSerial.
As an end-user there's not much you can do to resolve this as it is a pretty low-level issue.
The original ping library should work fine. I would recommend going with that.