Hi Mancha,
Unforunately, you cannot open IE without saving the file temporarily. Even if you did save the file and then passed the path to IE, your form would require Full Trust, which is a major pain.
The best idea I can think of is using the taskpane to render your output, and then have the user expand it out really big. We use XML+XSL transforms in the taskpane all the time and it works out rather nicely.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge