I use Firefox now but there are sites where you still have to use IE. If you use SP2 and actually read what you click BEFORE clicking it's not bad other than the lack of tabbed browsing and a startup delay.
Nah, if you go to any of the tech news websites they all have occasional articles about browser market share.
The consensus is Internet Explorer owns 90% of the browser market. Netcraft might track browsers; if they don't, I'm sure a quick Google search will reveal a few sites that track browser share.
If you run your own site, any of the log analysis tools will do this. I've managed a few sites in the past and based on my own experience, the 90% statistic is quite plausible.
I'm pretty sure only the EU version of Windows is the only one without Internet Explorer bundled with the operating system, if that's what you're talking about, a part of Microsort's antitrust settlement with the EU. They made no such concessions in other markets.
In any case, most corporate sysadmins will probably still install IE, simply because it's the browser compatible with the highest percentage of websites. Perhaps not poor reasoning, but that's probably why they'll do it.