You have a specific problem. Here's how to diagnose and resolve it.
Windows is misbehaving in ways that don't point at any specific application — SFC and DISM are the two built-in tools for finding and repairing damaged system files, and they work together, not as alternatives.
Windows won't let you log in and shows this specific error — almost always a corrupted user profile registry entry, with a fix that doesn't require deleting your files.
Boot times creeping up over time usually trace to a specific, identifiable cause — too many startup programs, a failing drive, or a driver delaying boot — not general 'Windows rot.'
A Time Machine backup that takes hours, or seems to hang at 'Preparing Backup,' usually has an identifiable cause — here's how to find whether it's the first backup, local snapshots, or something else.
A Mac that won't get past the Apple logo, or keeps restarting in a loop, has a specific, ordered set of causes — here's how to work through them from least to most invasive.
The app isn't actually damaged in most cases — this is Gatekeeper's quarantine flag reacting to how the file was downloaded, and there's a legitimate, safe way to override it for software you trust.
Ping by IP works but hostnames don't resolve. Here's a systematic path through resolv.conf, systemd-resolved, and nsswitch.conf to find where resolution is actually breaking.
An application errors out with EMFILE or ENFILE, even though the system clearly isn't out of resources in any obvious sense. Here's how to find and raise the actual limit involved.
Load average is climbing but top shows plenty of idle CPU. This almost always means processes stuck waiting on I/O, not a CPU problem — here's how to actually find which one.
A command touching an NFS-mounted path just hangs forever instead of erroring out. This is expected default NFS behavior, not a bug — here's how to diagnose it and when to change it.