Inclined to agree with doing away with Norton. (There is a removal tool available from symantec which should be used following the uninstall.) But as to why this has suddenly started when it worked well before? No idea, sorry. Would suspect some kind of malware as possible. and a scan with a good "second opinion" scanner such as Superantispyware, and/or DrWeb's Cureit should at least put that question to bed.
I found when I had Norton on a brand new PC it worked fine for a time (albeit a bit sluggish) but then started exhibiting slower and slower behaviour. So maybe it is Norton.
Good AV alternatives (free) include Avast, and Avira, and for antispyware Superantispyware, Threatfire, and MalwareBytes, Firewall alternatives include Online Armour and Comodo.
There are plenty more.