I defrag my C drive, but when I click Analyze it says that I should defrag the drive,
I've tried it three times .
Can you tell me how to repair this.
Thank you

You have some type of registry corruption within windows. The easiest solution would probably be to just reinstall windows.

Before you go to the trouble of a reinstall.

Try downloading, diskeeper Lite it's free, just search Google for it.

And also try Perfect Disk, free 30 day demo.

Use both see what they tell you 1st.

Also run chkdsk from start/run
And also chkdsk /f from the same window.

Also scandisk may help and any number of other options, besides a big reinstall.