On the client machine:
1. Go to Start > Settings > Control Panel
2. Open Administrative Tools
3. Double Click on the Microsoft .NET Framework Wizards shortcut icon (the version may be 1.1 or 2.0 depending on which version is installed)
4. Now the .NET Wizards little window opens.
Double click the Adjust .NET Security
5. Then a window pops up with two options "What Would You Like to Modify?"
6. Choose the first option "Make changes to this computer" and click Next
7. Next to Adjust the Security Level:
Highlight 'My Computer' and then raise the bar level of trust to Full Trust.
8. Click Next and Finish.
9. Then try opening the Symantec Endpoint Encryption Client from Start > Programs > Symantec Endpoint Encryption Client and it should work.