GUI quits working

On 3 machines now, after CIS has run for over 24 hours I find the GUI won’t appear no matter what you click on. There is a block with its name on the status bar of Windows XP Home (both machines) but it won’t “restore” to a window. If you try to restore it several times it finally quits responding altogether. CIS functions seem to be working fine, its just the GUI that won’t come up. Since this has occurred on two Windows XP Home machines I assume this is some kind of bug. A reboot of the machine does restore normal operation.
These 2 machines are set up really differently, but both have 3gb memory available to WinXP.
The 3rd machine, a Toshiba Laptop, runs Vista, and it did the same thing after running 2 days.
I’m not sure if this is something in Defense+ thats hanging, of if its something to do just with the GUI.
Any ideas?

When you see the button in the Task bar do you see cfp.exe (client or GUI) and cmdagent.exe running in Task Manager?

Are there programs that run in the background on all three machines? What other security/network related programs do you have running in the background?