Here is a tutorial on our website about installing Inventor, which does address the flash player issue. You basically have to uninstall it using the uninstaller from Macromedia's site (link in the tutorial) since it doesn't even show up in Add/Remove Programs. Then, finish the Inventor installation, and when Firefox/IE prompts you to install flash player the next time you visit a flash page, install it as usual, and Inventor still seems to work fine.
http://www.andersonrobotics.com/www/wiki/CadSetup