What OS are you using ?
With Windows XP, I used to do a Windows File Protection Scan,
1. Place the OS disc in the CD drive,
2. Open the Command Prompt, (type cmd in Run, then Enter)
3. Type "sfc /scannow" & press enter
4. Windows file protection comes up & starts to scan & says, "Please wait while Windows verifies that all protected windows files are intact in their original versions",
there may be more modern ways now,
Cheers
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