Application drops off the desktop and task bar

I have a new load of Windows Xp Pro. The users applications that are running will just disappear off the desktop and the taskbar by themselves. The Task Manager will not show them as a running application but will show the processes still running. There is now way to get it back except to kill the processes and restart the application. Any Ideas? All screensavers and powersavers are turned off.The desktop will just be idling there and they will drop off. But the wierd thing is, Outlook never goes away while the ohters do. Our databse program, Excel, etc all go away. Help!!! It was a format and a brand new clean load, after an upgrade from NT did the same thing.