I'm usually using Thunderbird, but for testing purposes have to start Outlook every now and then. Afterwards, Thunderbird informs me that it is no longer the default client, which it should have remained.
How to stop Outlook 2010 from being so insolent?
edit In fact, Outlook does not change any of the settings I checked so far (it doesn't re-register files, and mailto: links still open Thunderbird, so I'm rather confused at what Thunderbird considers to be modified by Outlook
