some good advice already given
its the same story with the windows OS, almost all the bad stuff is targeted at windows, because 90% of pc's run it and there appears to be no option otherwise, and anything like nOrton that very often comes bundled in a new pc and so is almost epidemic in its use, is going to come under the same fire, but you do have a choice here, so make a wise one. Dump nOrton.
If you have a router firewall corectly configured you will be fine with just the xp firewall enabled, (although personally, when using windows, I use Kerio).
Anti-virus: Big choice, but AVG Free is pretty good: http://free.grisoft.com/doc/1
or Avast Personal, also free for home use: http://www.filehippo.com/download_avast_antivirus/