I realise that you shouldn't disable your resident virus-protection whenever, but perhaps you should try that now and see what happens? To my knowledge, I haven't had a significant performance-drop with AVast active, but most people on gaming-forums suggest deactivating resident virus-scanners (and make sure you're not connected to the internet) when it's at all mentioned.
If you're considered an 'advanced' user, you might make some progress by having a look at your registry. Needless to say, be careful in there if you try this.