Norton Antivirus needs reboot to open

I maintain several Windows 7 pcs and have Norton Antivirus 2011 installed on them. Sometimes, but not always (of course), I cannot open Norton unless I reboot. Sometimes I even have to uninstall and reinstall NAV. Anyone had that problem?

How many computers do you have the problem with?

3. What they have in common is that they also have Clean Slate on them. Of course Clean Slate is disabled when I try to run Norton.