I think my computer is hacked by my friend.he was working on my computer and I think he has left some program on my computer by which he can get info about all my usage.How can I confirm it.. please help and what can I do?

Run an online virus scan

Go through this steps.
I think my computer is infected or hijacked. What should I do? Security - dslreports.com

I would suggest downloading Malwarebytes Anti-Malware 1.34, a free program that checks for spyware. Check for updates before running a scan.

Malwarebytes Anti-Malware - Free software downloads and reviews - CNET Download.com

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