Most of the laptops I work on that have overheating issues either have fans that look like dryer lint traps or that they were found in a sand dune on Tattooine.

I'm exaggerating (sometimes they're that bad)... Honestly, it doesn't take a great deal of caked on dust to affect a fan's performance.

It's uncommon that the fan itself has gone bad (unless you own a Sony VGN-CS series Vaio), though it is somewhat rare... and even more rare for the heat sink to need replacing.