A lagging PC is the result of a number of system issues, such as data corruption, bad files, low disk space, and bad software. You can use a few Windows tools to speed up a slow computer. Do a little backtracking to find the problem.

Run Disk Defragmenter
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Press the Windows button. Click "All Programs" and open "Accessories".
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Right-click "Command Prompt". Choose "Run as Administrator".
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Type "defrag X: -v" in the prompt, X representing the letter of your hard disk. For instance, if you want to defrag drive E type "defrag E: -v" in the prompt. Press "Enter".
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Wait for the defragmentation to complete. Reboot your system once it is complete.
Run Disk Cleanup
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Press the Windows button. Type "Disk Cleanup" in the "Start Search" field. Press "Enter".
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Click "My Files Only" when the Disk Cleanup Options prompt appears.
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Choose the drive and click "OK". Allow the tool to scan for possible space to free up.
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Place a check by the files you want to delete. Click "OK".
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Click "Delete Files" when the "Are You Sure You Want To Permanently Delete These Files?" prompt launches.
Scan for Harmful Software
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Press the Windows button. Click "All Programs" and open "Windows Defender".
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Click the "Scan" option toward the top of the window.
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Choose the repair option once the scan is complete. Reboot your system once the repair is complete.