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Why is my CPU at 100% when I'm not doing anything?

Updated November 2, 2026 · 3 min read

You are not doing anything, yet the fans are loud and Task Manager says the processor is working hard. Most of the time the cause is ordinary and temporary. Sometimes it is a program that should not be on the PC at all. Here is how to tell which, starting with the most common causes.

First, find the busy program

Press Ctrl + Shift + Esc, click More details if you see a small window, and click the CPU column on the Processes page to sort by usage. The name at the top is your starting point.

Normal causes that go away on their own

• Antimalware Service Executable. Microsoft Defender scanning. It runs heavier right after updates and when the PC has been idle. Let it finish.
• Windows Modules Installer Worker or TiWorker.exe. Windows installing or cleaning up updates. It can take an hour after a big update.
• Microsoft Windows Search Indexer. Building the search index, especially after you add many files.
• Microsoft Compatibility Telemetry. A periodic Windows diagnostic job. Annoying but harmless and short.
• OneDrive, Dropbox or Google Drive. Syncing a large batch of files.

The common thread: these have recognizable names, live in C:\Windows or C:\Program Files, and settle down within the hour.

Causes you can fix

• Browser tabs. A single tab with a video, a heavy web app or an ad-filled page can hold a processor core busy. In Chrome and Edge, press Shift + Esc to open the browser's own task manager and see which tab is responsible.
• Too many startup programs. Task Manager > Startup apps (the Startup tab on Windows 10): disable the ones you do not need running all the time. Your PC will start faster too.
• Heat. A laptop on a blanket or a desktop full of dust gets hot, and a hot processor slows itself down, so ordinary work takes longer and the fans run harder. Clean the vents with compressed air.
• An old driver or a stuck program. If one ordinary program is always at the top, update it or reinstall it.

Signs it is something that should not be there

Be suspicious when the busy program:

• has a name you do not recognize, or one that copies a Windows name with a small change (svch0st.exe);
• has no publisher listed (right-click it, choose Properties, then Details);
• runs from AppData, ProgramData or a Temp folder (right-click it and choose Open file location);
• keeps the CPU or the graphics card busy for hours, every day;
• stops the moment you open Task Manager and starts again when you close it.

That last one is the classic behavior of a hidden cryptocurrency miner, which is the most common cause of a PC that is busy for no visible reason. Here is how to confirm it.

What if Task Manager shows nothing, but the fans are still loud?

Two possibilities. The busy program may be pausing while Task Manager is open; the idle test in our miner guide gets around that. Or it may be the graphics card rather than the processor: right-click a column header in Task Manager and add the GPU column, then check the Performance page for your GPU's graph.

When to get help

If you have found a program that should not be there, do not stop at deleting it. Something put it there and something restarts it. Our removal guide walks through the restarters one by one.