Thought I'd post this link as it was annoying me and took a little while to find a straight forward explanation.

I've just recently moved over to Windows 7 and decided to go for the 64bit version, using Visual Studio 2008 I find that I can't edit whilst in debug mode.. all something to do with Visual Studio not being a true 64 bit app.

Well this helpful article told me all I needed to do to get this working again, just changing build configuration and adding a new active solution platform of x86.

Full details can be found here, well worth a read:

http://blogs.msdn.com/habibh/archive/2009/10/12/how-to-edit-code-when-debugging-a-64-bit-application.aspx