Please Help Restore My System

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? 

 Thanks for your concerned and contribution. . It displays not responding with the error warning "LSASS.EXE".
I first notice the problem when trying to copy an audio cd to windows media library.

Hmm, ,
   LSASS is a Windows component shown in error messages, often due to a virus infection such as Sasser. It exploits a recently patched vulnerability in lsass.exe. major infections would cause your computer to shut down in 60 seconds. but u didnt mention that. wot u can do is copy the error message and past it in a google search. you'll surely get som helpful results.

tell me your progress,

You could start by getting a good antivirus program like Kaspersky or Eset NOD32. A good numbers of worms and spyware imitate the names of standard Windows processes like LSASS.EXE and SVCHOST.EXE and your Avast may not be able to detect them.