i have a 17" mbp that's about 4 years old. lately it's been running slow and the fans have been working hard. there is plenty of free space on the hard drive so that's not an issue. the logic board was replaced last year. yesterday i got a blue screen when it started up. it went away eventually, but it sure scared the hell out of me. hard drive s.m.a.r.t. status is verified and i don't hear any loud clicks. i am backing everything up now, but should i be worried?