It's highly likely that the drive is on the way out. These things don't last forever (Google MTBF), and 8 years is aeons in computing!
Fortunately, hard disks are extremely cheap these days, and can be easily swapped. Essentially, you need backup software which is capable of backing up the entire drive and then restoring it to another one.
I use this:
http://www.amazon.co....qid=1269354750&sr=8-1
I do an incremental backup every night and a complete system backup once a week.