You can't just open System Properties applet and click OK out of it--you need to actually make some sort of change for it to resave the value. Just make some bogus change (such as adding an "X" to the very end of the path), click OK (actually, click OK twice to be sure; I didn't check to see when
System Properties saves changes). Then go back in and undo the bogus change.
If you want to be sure, open Regedit and navigate to this key to make sure the Path value is of type REG_EXPAND_SZ:
[copy or print