I have a problem that started today, where trying to run any of the 3 versions of Opera I have installed causes Internet Explorer to start instead.
The Opera versions I have installed are 9.64, 10.10, and 10.63. My OS is Windows Vista with the latest critical/important updates.
The behaviour is the same regardless of whether I double-click a shortcut to Opera, double-click on Opera.exe in one of the install directories, or run Opera.exe with the full path from Start->Run.
The only work-around I've found is to right-click Opera.exe or a shortcut, and click "Run as administrator". This starts Opera, and it appears to work normally.

Opera seems to be the only program so affected. I've checked Firefox and a few other (non-browser) programs, and they work normally.

When IE starts instead of Opera, there are always two instances of iexplore.exe started, and they both crash before I can do anything else.
IE also crashes if I try to start it from its own shortcut, but I don't know if that only started today, since I hadn't used IE for a few weeks.

Does anyone know what might cause this and how to fix it?

It's possible my PC has a virus, but BitDefender (for which I have automatic updates enabled) hasn't detected anything.