is this a virus?

There's this 2 windows that automatically pops-up. the 1st window says "your pc is infected with virus..." and tells me to download something to prevent the virus from harming the system. The 2nd window shows AVG is scanning my computer and has detected an infection.

Is this thing a virus? because, you see, I've scanned my system, and there's nothing in it. Is it possible that my browser is the one sending those stuff? how can i get rid of it. and by the way i'm not using AVG as my virus scanner.

Hello.
Yes, it's an infection.
Please read here and post a Hijack This log.

http://www.geekpolice.net/malware-removal-hijackthis-logs-f11/read-this-before-posting-t3821.htm

Moved to malware removal.

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