Chances are Netscape is defined as your primary browser (that is, associated with .htm and .html files by default).
Easiest way to correct this "problem", without reinstalling IE, is save a html file to your hard drive (doesn't have to be anything important). Next, in Windows Explorer, while holding shift (either one) down, right click on the html file, in the resulting context menu, select Open With.., in the resulting Open with dialog box make sure "Always use this program to open this type of file" is selected, then highlight IExplore, and click .
As to your second question, it shouldn't cause any problems to use Netscape Mail. But, then again, we're talking about Micro$oft products.