If you are installing our products for Clarion 6 or older, then the reason is most likely that the WIN.INI was not updated correctly when you installed Clarion. This happens on computers with User Access Control (UAC) turned on because the Clarion install is not UAC aware and is not elevated by default. This causes it to write to the WIN.INI in the virtual store rather than in the windows folder. To fix this you need to download and run a program that Friedrich Linder at Lindersoft created to fix this problem, called Clarion 6 Vista Installation Fix. This fixes the WIN.INI in the windows folder and sets the PATH environment correctly.
This also applies if you are using Build Automator to compile with Clarion 6 and older.
If you have this problem in Clarion 9, then please visit this link http://www.lindersoft.com/forums/showthread.php?t=40434 and download a tool that Friedrich Linder of Lindersoft has made available that will detect any problems there may be with the Clarion 9 install.
If you have this problem in Clarion 7 or Clarion 8, then the first thing I would suggest is to reboot.