Based on your question, you can find information about Impact.ttf from Microsoft site itself. See Impact.ttf 2.3 and Impact.ttf 5.0. The font isn't a copy right with Microsoft either. It just tells you that following Microsoft products are using this font, and they would have contractually worked out the mutual license to it. The font is owned by Monotype.com which provides many such fonts and customization service. Also, you can find out that the same font is available for sale at $49 from fonts.com. So it is clear that it is definitely not a free for all font. There are numerous sites which allows for download - i think it is clearly piracy!
Now having said that, the reason why I wront this answer is to clarify two important things.
First: Nothing really conflicts any other license. So if you download & bundle Impact.ttf and distribute it, you are clearly violating copyright. Whether you distribute it like pirates or under MIT or other free license or under commercial or binary license.
Distributing anything for which you don't have license to distribute, let alone license to use it, is clear violation of copyright and it has nothing to do with license under which you distribute other parts of your own work
Second - BUT, if you distribute any thing where by you dont have to bundle Impact.ttf in your own distribution, and you can instruct users to download and link with your font, then clearly you are not distributing the font. End user who need to use it, need to pay and use Impact.ttf or they can be directly available in their system by the virtue of the fact that Windows Xp, Vista, 7 & 8 provides to all end users throught Microsoft's own license.
All you need to do is, do not bundle Impact.ttf in your distribution but rely on system dependency and in case it is not available in system there could be system's own fall back. Then it is perfectly legal for your software to utilize it on client's desktop.
Again, MIT license or any other license has nothing to do with license of the font!