Programmatically change default web browser

My vista laptop is used by myself and one other user.

I'd like to change the default web browser programmatically for each
user at logon.

I prefer IE and the other user prefers SlimBrowser.

Is there a way to change this, as part of a logon script(?), as a user
logs on to the machine.

Setting the default browser manually sets it for every user.

Any help would be great.