one of your brush xml files got corrupted... I remember when it was happening with older painter versions (like 7 or 9), simply deleting it solved the issue. painter was creating a brand new xml file for the same brush after relauching, but if it was custom tweaked, all customization was obviously lost.
try this: open (your name) -> library -> application support -> corel -> painter 12 -> (name of your current workspace, or "Default" if you aren't using a custom one) -> brushes -> pencils -> delete the "mechanical pencil.xml" brush variant
you will have to do it twice, with brushes -> painter brushes and brushes -> painter 12 brushes, because I'm not sure what exactly brush library you have loaded, and I'm not sure which file is causing the trouble.
//upd. sorry, I haven't seen that you've tried the factory reset thing already.
try removing the xml file and see it helps anyway. "mechanical pencil" isn't anything unique, and if you have an older painter version around, you can copy the xml file manually later.