Please review KB58826 and check if the symptomps listed in Problem 3 and Problem 4 apply in your case. If yes, then follow the steps in the solution section to fix the issue.
If that does not cut the ice, please check if On-Access Scanner is enabled (a green check mark on the top right hand corner of the PortalShield console indicates that it is enabled).
Examining the windows application and system event logs would help identify the specific nature of the issue.
Hope that helps.