Error 8504,4 in NIS 2009 twice now

Twice in the last 3 weeks I have had to use the Norton Removal Tool and reinstall NIS 2009.  Both times occurred after PC Tools Spyware Doctor found and removed a virus / trojan that NIS did not catch.  (Maybe it was already shut down?) I have used these two programs together for a long time without a problem.

I must decide in the next couple of months whether to continue using Norton or switch to something else.  I have been a satisfied Norton user for many years, but do not have the time to continue to uninstall / reinstall the program, plus monitor to ensure that it is actually open and not leaving me running bare.  I would appreciate any ideas on why this has happened and how to prevent another occurrence. (I am using Windows XP SP3).