I've had various issues like this in the past and have had to do a complete uninstall of the LANDesk software on the client. The uninstaller, UninstallWinClient.exe, located in LANDesk\Management Suite\ldmain, is what you want to use. I typically copy the file to the client in question and run it either from the GUI or command line (go in remotely via psexec).
The file has some switches you can pass to it. From the command line, run "uninstallwinclient.exe /?" to see what they are. I typically run the following:
uninstallwinclient.exe /UI /FORCECLEAN /REBOOT
After that, reinstall the agent and see if it is good. This probably isn't the quickest solution but it's definitely very thorough.