Unfortunately, the Build Events just runs commands - it isn't a full command shell where you can call cd, etc.
One option would be to make a batch file that looked like:
cd %1
nmake
Then call your batch file directly:
myBuildBatch.bat "${ProjDir}\Windows"
This way, it'll execute inside of a batch file, which gives it a context where you can use cd.
Reed Copsey, Jr. -
http://reedcopsey.com