what does /defaultc.aspx mean on my address?

Hi.
I realize this question has little to do with windows vista, however I am so very new to computers and this is the only help site I belong to so I thought at the very least someone could point me in the right direction.
Here is the problem: once I log on to my computer and click on the explorer icon to get me to the internet, the address that comes up in the address box is:"
MSN.com /defaultc.aspx " which is different from what it was the last time i really looked at it. It use to just read MSN.com . Can anyone tell me why it would change, or if it might mean i have some sort of bug or spyware? I would also appreciate an address to a site that addresses this kind of thing.
Thank you
grace

Interestingly, you can type defaulta.aspx, defaultb.aspx and defaultc.aspx (but not defaultd.aspx). I would assume the website was working with different versions of their homepage. No bug.

Thank you for the response lemur.