The summary.
RESTART WEEKLY, AND FIRST WHEN SOMETHING IS WRONG
It resolves a substantial proportion of problems in seconds.
WHAT ACTUALLY MAKES AN OLD COMPUTER FASTER
Free disk space, fewer startup programs, and more memory.
Never install software promising to speed up or clean your computer. Most do nothing, and some cause harm.
ENABLE AUTOMATIC UPDATES, AND CHECK MONTHLY THAT THEY APPLY
Machines stop updating without anyone noticing, and a system no longer supported is a permanent vulnerability.
HEAT AND DUST CAUSE MORE FAILURES THAN ANYTHING ELSE
Keep vents clear, keep machines off soft surfaces, and use surge protection.
For a spill: power off, disconnect, turn upside down, and do not switch it on to check.
A SWOLLEN BATTERY IS A SAFETY MATTER
Stop using the device and have it replaced.
THE FOUR DIAGNOSTIC QUESTIONS
What exactly happens, when did it start, what changed, and is everything affected or one thing?
Change one thing at a time, and record the exact error message.
BEFORE ANY REPAIR, BACK UP
Repair frequently means replacing the storage, and your data with it.
Check who repairs a device locally before buying it.