Wow that was a long title but hopefully it will help out when people run into this problem.
I ran into a small issue when installing the June 2010 Windows Phone 7 Beta tools (yeah I know I'm 2 months late...been busy!) in that Symantec Endpoint Protection's Tamper Protection tool kept blocking the install when it got around to the install portion with Imagegen.exe
Just turning off Symantec Endpoint Protection services isn't enough, you actually have to go into the client settings and uncheck the Tamper Protection setting.
If you already started the install and ran into that issue, be sure to do an uninstall/reboot after turning off Tamper Protection before trying to re-install the Windows Phone 7 tools.
Oh and be sure to turn all that stuff back on when you're done installing!