Measure the slowdown before trying fixes
“Windows is slow” can describe several different failures. A PC that takes five minutes to start is not the same as one that becomes unresponsive after opening a game, reaches 100% disk usage while idle, or loses speed only after it gets hot. Begin by noting when the problem appears, how long it lasts and whether it started after an update, driver, app or hardware change.
Open Task Manager with Ctrl + Shift + Esc and watch the CPU, Memory and Disk columns for a few minutes. Short spikes during startup, updates or security scans can be normal. Sustained high use, repeated freezing, abnormal fan noise, drive warnings or worsening performance needs closer investigation. Do not end unfamiliar system processes only because they appear near the top of the list.