Like any Windows XP machine, the SureDesk™ benefits and sometimes suffers from Memory Leakage. This can cause an XP computer to run slow or be intermittently non-responsive.
A likely fix for these slowdowns is to restart the desktop.

The SureDesk™ will shut down and you will have to wait a few moments for the SureDesk™ to restart before you can launch the Citrix viewer again.
Also if you notice that Windows updates are available to install, installing these updates will help as well.