While the usual is malware and other infections a few recurring repairs that make you wonder why happened last week so I'll share them so you can add your stories too.
1. An old horse of a laptop and reloading it to factory condition didn't bring it back to it's former speed.
What was it? The battery was expired. The big battery would not charge and on a guess we popped in a new battery. The machine is working well now.
Cost? Amazon battery to the shop was 33 bucks.
2. A newer laptop was slow and once in a while randomly locking up. The usual malware scans and canned air work was done but still the odd lockup. A hardware diagnostic was performed, all passed but out of the blue the HDD spun down and it bluescreened. Again the HDD passed its maker's test.
What fixed it? A new HDD was fitted. The old HDD contents were cloned to the new drive and it's been fine since.
Cost: New 500GB laptop drive under 50 bucks.
In both cases the owners questioned the repairs made but the proof was there for them to try it out.
Note: Edited to share the cost of the parts alone.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic