Norton Anti Virus Disabled at Start Up

I am running Norton Antivirus 13 on Windows XP.  Suddenly a few weeks ago, upon start up my anti virus was disabled and could not be enabled.  I had to uninstall with the Norton Removal Tool and Re install.  That would solve it but only for a few days.  After turning off the computer at night and turning it on in the morning it once again was disabled.

 

I tried the Norton Power Eraser and the Microsoft Safety Scanner and neither detected anything nor did my Norton Anti Virus after a full Computer Scan.

 

Any ideas for why this is suddenly happening and how it can be stopped?