Can't find Team Foundation Server exe

[jjkboswell]

I've got two build machines, and one of them happily finds tf.exe for my Team Foundation Server step, but the other doesn't (same script):
Unable to auto-locate Team Foundation command-line executable; specify the full executable path + filename on the Options tab

On the machine that succeeds to run the step, it outputs the following for the command it is calling...
"c:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\tf.exe"

And on the machine that if fails, tf.exe is in exactly the same folder.

I checked my global macros, and DEVSTUDIO_NET_DIR is correct ("c:\Program Files\Microsoft Visual Studio 9.0")

So how does VBPro auto-locate the exe and why is it not finding it? What am I missing on one machine but not the other?

James