Whenever I try build Expression Blend on a machine for the first time, I always encounter this error when I hit F5

The specified solution configuration "Debug|HPD" is invalid. Please specify a valid solution configuration using the Configuration and Platform properties (e.g. MSBuild.exe Solution.sln /p:Configuration=Debug /p:Platform="Any CPU") or leave those properties blank to use the default solution configuration.
Done building project "Name Of Your Project.sln" — FAILED.

It goes without saying that it is irritating that you cannot use the application from the word go. I have encountered this error on both 32 and 64 bit windows versions, and resolve the issue thus;

Go into the advanced system settings (I am using windows 7)

Select the advanced tab, and click the “Environment Variables” button

Delete the “Platform” variable

After restarting Expression blend, you should find that you can F5/Ctrl+Shift+B to build your solution.