The most common reason (and actually the only reason known by us) is conflict with some other software. It could be anti-virus, anti-spyware, network filtering or any other security software that is blocking iNet Protector either due to poor programming or because it somehow thinks that iNet Protector might be harmful.
Here is the drill:http://kb.blumentals.net/kb/index.php?V ... EntryID=54
I know this might be difficult to do, but there is no other way.
You should also check that:
1) IProtectorService is set to run on start-up and not manually (normally users should not be able to disable the service, but if you have left security hole allowing them to do so, it might be possible for advanced user to do this). Our knowledge base has an article about securing against this.
2) iNet Protector is not currently compatible with 64-bit systems, check if they have 32-bit Windows installed.