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'.