Kernels?

This is the nearest heading I could find for this question so sorry if it's in the wrong place. I've recently switched anti-virus from AVG to Avast. I thought I'd successfully removed AVG and Avast has installed ok. However, whenever I switch on an error message comes up 'Could not initialize the AVG Anti-Virus Kernel Interface' and AVG is still showing as one of my programmes. When I click on remove it says 'Installation error' and stays there. I should add that I switched to Avast because, having removed AVG 7.5 to get AVG 8 I found after several attempts that AVG wouldn't install and I couldn't get AVG 7.5 back even after doing system restore. It doesn't seem to be causing any problems with Avast but I'd just like to get rid of AVG entirely. I've tried CCleaner but it's still there. Any ideas anyone?