Mastercook can work with Vista if you do a few tricks to tell Vista that the data files reside within "Program Files" (directions can be found here >> http://mc6help.tripod.com/mastercook_and_vista.htmn). This will let you get to your recipes so you can begin the conversion process to BigOven. At any rate, I have been using MC8 and finally figured out how to export multiple recipes to the correct format and then import them all at once into BigOven. Here's what you do once you can get to your files (seriously, follow the directions in the link - they work):
In Mastercook, choose "File\Export and Mail" from the drop down menu. When the new window opens, choose Folder from the tabs at the top of the window and select the folder you want to put the recipes/cookbooks in (I created a special folder so I could find everything easily). Under Text Options, click on a format that gives you either mxp or mx2 files (mine was MC 4 format only). Then in the "Drag and Drop from the MasterList" box, use the Masterlist button to the right to open the MasterList window. Once there, find the cookbook or recipe you want to export and drag it into the "Drag and Drop" box. I chose all recipes from a cookbook so I could speed the process. Next, in the very bottom of the export window, choose single file and give it a name (either the recipe name or a cookbook name). Then click "Export".
After you have done all of this, all you need to do is go into BigOven and walk through the import process. I did entire cookbooks with over 1,000 recipes in just a few minutes. Once I figured out how to change the MC file format in mass, it was easy. Good luck getting all of your recipes out!