Often it's the USB GPS stick. If you bought the GPS in a package with S&T, you could try exchanging it for another set. Also try connecting it to a different USB port.
There's a three-page thread where people (many who are very technically-experienced) try to diagnose crashes:
http://www.laptopgpsworld.com/405-microsoft-streets-trips-crashes-unpredictably
About half of the people posting on that thread seem to have tracked down the problem to hardware. The other half seem to have gotten stuck, because it was a difficult settings or driver issue (and those are never easy to work out).
S&T appears to be less tolerant of malformed input data than other programs. You can either switch to another program, or fix whatever hardware problem was causing malformed input.
There are a number of other programs you can use. On this forum, Garmin Mobile PC is the second-most-popular, after Streets & Trips. Go back to the main page of this forum, and you'll see forums for the other popular software packages. In each of them, there'll be a review thread near the top, exploring the capabilities of the program.