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 (http://spywarefighter.blogspot.com/2008/09/trend-micro-housecall-online-virus-scan.html)

Go through this steps.
I think my computer is infected or hijacked. What should I do? Security - dslreports.com (http://www.dslreports.com/faq/8428)

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 (http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html)

Ill sugest downloading super anty spyware it's the best at taking away those spyware and if you want to spend some money get the pro editon then it will protect your PC in real time.

Download at SUPERAntiSpyware.com - AntiAdware, AntiSpyware, AntiMalware! (http://www.superantispyware.com/)