"It runs in the background." --- Ambimom
What do you think Norton Anti-Virus was doing? Does it run in the foreground? Sheesh!
"... this morning there was an update that I was instructed to load ..." --- Melissa
That would be good information to share, since Patch Tuesday occurred ... on Tuesday.
Norton did its job, advising the user of an other-than-normal condition. Just like the Viagra commercial warning, if the high CPU usage lasted longer than a few minutes, then there should be concern.
Curiously, why did mrt.exe kick in when one has NortonAV installed and running? Does one have more than one security program running concurrently serving the same function? I use Norton Internet Security, and mrt.exe only runs when I explicitly run the Malicious Software Removal Tool, kb890830, made available on Patch Tuesday.