Fungll, the problem with ALL of these products is that they are reactive... that is to say they cannot protect against something they don't know about.
So some poor person somewhere has got the bug/adware/trojan/dialler/worm/virus then submitted it to the companies who then write a routine to defend against it and put it in their updates.
This is why there are sometimes 3 or 4 updates in a single day.
Norton is primarily a virus checker. Adware/Spyware is something else (and it depends on who you talk to as to what constitues adware/spyware). Some programs you want won't run without it. As technicians we use a number of tools because we know that not every product works the same way or looks for the same things.
You want to run an online virus scan every now and then (free at housecall.trendmicro.com or panda) and a spyware scan such as spybot or adaware.
You must not have more than one active virus checker at one time, but you can use several different spyware checkers. Alot of spyware checkers are in fact spyware themselves...
spywarrior here gives perhaps the most definitive list on the net.