How to fix "sharing violation" message when editing hosts file?

I'm running textpad as administrator. When I try to save the changes, I get a message "Encountered a sharing violation while accessing C:\Windows\System32\drivers\etc\hosts."

I checked using Process Explorer and the only handles it finds that reference hosts are "System".

How do I get around this?

I have attempted both replacing the hosts file by either editing it, pasting the new version in and saving it, and by copying the new "hosts" file as administrator. Latest attempt got a message "file is open in system". I'm running ZoneAlarm for security.