Before you nuke and pave, try this. Nine times out of ten it's one of four things: too many startup programs, a full or dying drive, a browser that has eaten all your RAM, or malware. Let's walk through them in order.
Open Task Manager (Ctrl+Shift+Esc) and click the Startup apps tab. Anything listed as High impact that isn't something you actually use at boot — disable it. Slack, Spotify, Adobe Creative Cloud, manufacturer bloat (Dell SupportAssist, HP Touchpoint, Lenovo Vantage) — all of it off. You can still launch these apps manually; they just won't slow down your login by 90 seconds.
Next, check your drive. Open Settings → System → Storage. If your C: drive is more than 85% full, Windows slows down dramatically because it has no room to page memory to disk. Uninstall anything you don't use, run Storage Sense to clear temp files, and if you're still full, move your Steam library or Downloads folder to another drive. On older machines, if the drive is a spinning HDD, replacing it with a $40 SATA SSD is the single biggest speed upgrade you can make — it's not even close.
Browsers eat RAM like it's free. If you keep 40 tabs open, your 8GB machine is going to suffer. Try Edge with Sleeping Tabs turned on, or Chrome with Memory Saver enabled (Settings → Performance). Close extensions you don't actively use. If your machine has 8GB of RAM and you can upgrade to 16GB, do it — the sticks are cheap and the improvement is immediate.
Finally, run a scan. Windows Security (the one built in) is fine — open it, click Virus & threat protection → Scan options → Full scan. If you want a second opinion, Malwarebytes Free is the tool I reach for. It catches the adware and PUPs Defender tends to leave behind. Between those two, 95% of malware slowdowns get solved.
If none of that helps — and the machine is more than six or seven years old — it may just be time. A $400 mini PC today outperforms a high-end desktop from 2017. There's no shame in upgrading hardware that has earned its retirement.