what you do when your competer got affected by any virus attack?

Clean it. I am using different antivirus applications like AVG, Malwarebytes, Bitdefender, SuperAntispyware and Spyware Doctor. You may also try cleaning your PC via Bitdefender's online scanner or Trend Micro's Housecall. Good Luck!

Re-install is the best way I think. I used to do so, because as my friend says that if a PC has been infected by a virus. It means that the roots of the windows (OS) has been infected too.

It is always a good idea to keep your important data backed up on some secondary hard drive or on DVD/Cds . So all you need to do is reinstall the operating system and you are all done. No anti-virus solution can give you 100% security , there is no such thing as a silver bullet as far as virus /trojans/Malwares are concerned. I personally use Norton Corporate edition as my anti-virus but at the same time i take frequent backups of all my important data. Take care.

The first thing I do is delete my cookies, in fact I do that pretty regularly anyway. If you get a virus and manage to get rid of it, you are quite likely to pick it up again if you haven't deleted your cookies. I use AVGfree and Spybot to scan for viruses, but I'm pretty sure they miss a lot. Sometimes you can figure out what virus you've got and look for clues on the internet.

Avast or Eset Nod32 are my preferred antiviral program that will even block a website and create a pop-up message to inform that that site is known for malware. As previously mentioned, always update you anti-virus program and perform a deep system scan at least once a month and you should be fine. CCleaner is a good program to delete junk off of your compute (cookies, temporary files, etc.) that will help against corrupted cookies and improve your memory.