My solution: I tried a more thorough uninstallation using "Revo Uninstaller Pro 3.1.4", and when I reinstalled Wifi Guard afterwards, it worked. In case it is of any help, below are a few leftover registry values which the default uninstaller did not remove (I'm running Windows 7 64-bit).
Alternatively, simply use the portable version - it works just fine (both 32-bit and 64-bit) without any need of ever install or unuinstall anything.
