Please Respond To This

My system Started behaving strange two days ago. It's always slow when performing operations and hangs at-times showing not responding. Even when I want to shut down with no program running it still tells me "Not Responding" until I have to click on "end now". I use Win XP Pro with fir-wall on for most applications. I also have a running Avast home edition Antivirus. I am suspecting this could be a virus. What should I Do?