Okay, I can now run Avast again, and it seems it removed (quarantined, anyway) Win32:Cycbot-KI. I googled, and it appears to be a common issue when people remove this. I still can't run Explorer though, which is really frustrating/worrying. I'm getting the message that there is no issue anymore when I rerun the scannow prompt. Is it worth running again, do you think, just in case, or is there another fix I could have a shot at? I seem to be stuck between having no online access and having a trojan...
Many thanks again for your help!