I use 64-bit Win 7 for flashing. Just install the 64-bit version of WMDC. Also, when you connect your phone and WMDC asks if you want set up a new partnership, just select "not at this time" and it'll connect you as Guest. Then flash away. I don't think it even lets you install a 32-bit version of WMDC on a 64-bit install.
Here's a link to what you need:
http://www.microsoft.com/downloads/d...displaylang=en