While working with Visual Studio 2008 on a 64bit version of Windows you'll notice that you are unable to alter code while the project is running. For example after hitting a break point you often want to update the code before continuing the execution, but as soon as you try this a dialogue pops up saying "Changes to 64-bit applications are now allowed". I suspect this is because VS is still a 32-bit program, and thus cannot hook into your 64-bit process.
To work around it you can compile your project as a 32-bit program: