Almost certainly your HDD is the problem. From experience of exactly the same symptoms and then a fatal crash within days I suggest you take the following action:
Create an immediate backup of the entire drive, preferable onto an external drive or burn an ISO image to CD`s or DVD.
Obtain another HDD asap and replace the existing one then transfer everything over.You might consider buying a 2.25" external HDD Caddy to make the operation easier if you can`t transfer all your data to CD`s. (About £20)