The 'default browser', as I understand the concept, is the program started if you click on a hyperlink for a website. According to the file-type screen (in Windows 95/98) that's rundll32.exe shdocvw.dll,OpenURL %l, and I assume the dll knows what to run from the item in the registry that defines the 'default browser' you can set in IE and in Mozilla.
But I've never seen a browser (IE or Mozilla for that matter) starting automatically triggered by a dial-up connection. It doesn't make sense. It's quite possible you intend to use the connection to check your mail (with Outlook Express for example) or just want to go to ICQ or MSN to have a good talk with your friends, or want to run Mediaplayer to listen to some Internet radio.
The other way around, yes: you can set IE to dial automatically if it is started without a connection. And of course, you can put IE in the startup group (or Mozilla, or in fact any program or browser or document or web-address).
And you can put the dial-up connection in your startup group if you want to connect automatically at boot time (but then maybe you'de better take dsl or cable connection).
So - if it really is a startup problem, as you describe - the question is: can you disclose what runs at Startup (msconfig is the tool to use, even in Windows XP), and what Services are started?
If it isn't, are you sure it's really triggered by the dialup connection, if you click on it from My Computer?
By default,I mean that when I boot my PC and connect to the Internet, I would like Mozilla to open instead of IE 6.0 first. I have tried any combination of settings stated at this site and more,with no luck.
Maybe there is some association between IE 6.0 and my AT&T Dial-Up connection or maybe a Registry entry that I don't know about it.
I have Windows XP Home and I am not sure if Pack 1 is installed simply because soon after I installed the Pack 1,I restored my system to a previuos restore date.
Any other idea helpful or not,shall be appreciated.