When Data Execution Prevention (DEP) feature is configured to run for "all programs and services", attempting to implement a Citrix verification in TrueLog Explorer results in the error "Failure to perform OCR".  
Data Execution Prevention (DEP) was introduced in Service Pack 2 for Windows XP and Service Pack 1 for Windows 2003 and is set of hardware and software technologies that perform additional checks on memory to help prevent malicious code from running on a system.  

This feature can interfere with the 3rd party libraries used by Silk Performer for implementing OCR when be enabled for all programs.  To resolve this, enable DEP for essential windows programs and services only using the following instructions:

1. Right click on MY COMPUTER and select PROPERTIES

2. Click on ADVANCED

3. Click on SETTINGS in the PERFORMANCE section

4. Click on DATA EXECUTION PROTECTION tab

5. Ensure that the radio button "Turn on DEP for essential windows programs and services only" is enabled.
If that does not work, try disabling DEP by the command line as per this article.