Error 8506,421 & 8504,100

Having problems accessing Norton 360 on my desktop,  I don't use my desktop very much and often forget to manually run Norton. tried it today and not getting anywhere.  I cannot bring up the opening page I keep getting error reports.  I remember this happening earlier on this year when something was blocking access to Norton.  I managed to find it and delete it after which everything was fine.  The trouble is, I can't remember what I went into to find this rogue ?PUP?  I have windows XP on the desktop and Windows 7 on my laptop, so everything looks different.  Advice please.  And can you tell me what the above error number mean please?

many thanks

Freddles