When installing Spamfighter's web version (downloaded file name spamfighter_web.exe) you may receive the "Error applying transforms. Verify that the specified transform paths are valid." error on some systems. Before delving deep into the registry to fix dead *.mst references (which might or might not work), it might be quicker to download the full version instead of the web installer. Also check out the Spamfighter support pages. Hope it helps.
Roy Shakespeare: Thanks very much Dirk, I had problems installing the Spamfighter upgrade with a Windows Installer error message. I downloaded the 'full version' from your website and installed the updated programme without any problems.Brilliant!