Pascal writes about the joy of x64 Windows XP, and some drawbacks. One drawback is no Edit and Continue using the 64 bit CLR. You can, however, use E&C against the 32 bit CLR on a x64 machine. Here's how you do it:
You need to compile your managed assembly with a target CPU of x86. This will cause the 32 bit CLR to be used rather than the 64 bit CLR.
For a VB Project, right click on the project and go to Properties/Compile/Advanced Compile Options/Target CPU and set it to "x86".
For a C# Project, right click on the project and go to Properites/Build/Platform Target and set it to "x86".
If you want to (or must) use the 64 bit CLR, we unfortunately cannot provide E&C. In C# if you try to edit there is a dialog that tells you edits are not allowed . To prevent the dialog, disable E&C: Tools/Options/Debugging/Edit and Continue, and uncheck 'Enable Edit and Continue'.