Finally, more fear than harm : The IE issue I had came actually from a IE Tab bug who is using IE7's rendering engine while interpreting IE6 conditional comments and so, that's why the display problems in the login page.
I looked in the Start menu and I actually found Internet Explorer 7 in the applications list.
And for the icon on my desktop, I think it has something to do with the IE Tab bug and Multiple IEs. So, I just created a new shortcut to IE7 and then, I will temporarily deactivate IE Tab and use the real IE7 for tests. The login page finally looks OK under IE7.
Ubuntu Linux 10.04 LTS / Windows XP SP2
GIMP 2.6.8 (advanced), Inkscape (newbie)
XHTML/CSS (advanced), PHP/MySQL (medium)
Don't ask what free software can do for you, but what you can do for free software.