Your help is extremely useful, Milo - thanks again.
Have been working through the steps you nominated and am up to trying to remove something called 'Ask Toolbar', publisher Ask.com, which I certainly haven't purposely installed.
When I attempt the uninstall in Control Panel, I'm asked if I 'want the following program from an unknown publisher to make changes...', (Windows\Installer\174ed0.msi)
If I click yes, a message tells me all IE browser windows should be closed in order to complete the uninstallation, although none are open. This message keeps coming up and won't progress past that point.
If I click no, I get Error 1730, telling me I must be an administrator to remove the application. As far as I know, the account we use is designated as the administrator already, so again I'm unable to proceed.