Nigel
Please download and run ADWCleaner and post a copy of the report produced:
http://www.bleepingcomputer.com/download/adwcleaner/
I suggest you take the precaution of creating a system restore point before running ADWCleaner, to guard against unforeseen consequences. Select Start, Control Panel, System, System Protection, and click the Create button.
Uninstall and reinstall Microsoft Security Essentials.