I recently assisted a client who was having a problem with his Dell computer blue screening every time he ran a disk degfrag. I asked him to document the error code and then let me know what it was reporting. A quick check for the error code on the web revealed that this is apparently a well documented error which affect Dell systems (and perhaps others) and is the result of a bad driver from Intel via Dell. Below is the link to probably the most comprehensive review of this issue.