How long does a laptop last?
Toshiba NB-200 netbook for me... bought in 2010 and still going strong... despite a nurse spilling a drip all over it and having to dry it out over a radiator for a couple of days lol. Power button and a few F keys are sticky / non functioning but other than that the main machine itself works ok. Shame as I really miss not being able to disable the track pad with the F key when using a usb mouse... oh well..
Anyhow a lot of laptops start overheating and thats when people tend to scrap them. I knew an IT pro years ago who said he didn't like laptops because they overheat and become unreliable. He had no idea about taking them apart and cleaning out the cooling system / fins that are usually choked up with dust and stopping the air flow.
I took apart a friends old laptop a couple of years ago, I had to backup everything but to do that I had to leave it in the kitchen because the fan was that noisy. Got it apart, found a ton of people online saying it needed a copper shim and I ended up taking the cooling fan unit apart to find a carpet of dust the other side of the cooling fins. Cleaned all that out, reassembled, got rid of the virus's, reinstalled windows and the machine ran so quietly you had to struggle to hear it. I was well proud 
... until the owner told me a week later she'd not noticed any difference in noise at all 
6 months later I went round there to see it sat on top of the carpet... a long wooly carpet... blasting the fan away like crazy again 
So the moral of the story is that it depends how well you look after your laptop. If you look after it very well and don't let any bad happen to it then it could last well over a decade or you could trash it and it might be on ebay as parts / repairs after 8 months.
Similar fan running noisy with my Toshiba but caused by the heat sync becoming detached which made the whole thing run hot and fast. Squirt of heat compound and a refix and it's been fine since but as I was to find out later, apparently it was a stock fault on my particular model. Still humming away two years later glad to say. 
chkdsk simply finds (and possibly fixes) file system errors. It won't identify physically bad sectors and put them in the bad blocks list - this happens automatically on writes.
Must admit i thought it was scandisk that did it 
You may be right. It's a long time since I had need to run chkdsk. NTFS probably maintains an index of some start locations of free space, so it might just be checking that you don't have "hidden" free space.