Thanks to both.
Razzledazzle - my friend says he doesn't have a Vista system disk. Only a Dell recovery disk. I'm not sure whether it's possible to repair/reinstall just one Windows component from this. Do you know, please?
Brundle - I suspect this is going to be more than just re-setting IE as the default browser. It no longer appears on the list of installed programs, so my friend needs to re-install it, or at least repair it.
Any ideas please?