Well this could be an issue with my machine, but IE isn't on my list under "set your default programs".
It is there under "set program access and computer defaults", but selecting it was having no effect. In the end, the only way to reinstate IE as the default browser was to go back to a restore point before Opera 17 was installed. After that, I was able to re-install Opera 17 and de-select the option to set Opera as the default browser.
In the course of all this, I discovered that Opera 12.16 has the same install process, i.e. after you start the install you have to select "options" and untick the option to select Opera as the default browser. The difference is that if you forget to do this on installing Opera 12.16, you can change the default browser later, whereas on installing Opera 17, if you don't untick the box then you are stuck with Opera as the default browser for ever.
Oops, maybe I shouldn't use the phrase "stuck with Opera as the default browser" on an Opera forum 