When you view source in Internet Explorer, Notepad typically comes up. Now, I know you can customize this, and that's great. The problem is, why should it be a customization you have to make? I think IE should know enough to format and colorize the source.
On a side, but related note, if you want to beat Firefox in this respect, make the damn thing editable, too. I hate that about firefox. While its color coding and inline search make the source viewer great to work with, there are times I want to edit it -- even if it's a small format change to make it easier to read. I get aggravated every time I try that.