What to upgrade?

What I like to know is how are you supposed to know what to download or upgrade too?
Almost everything you do now days have some sort of anti-virus and spyware with it.
I.e.: Internet explorer 8, windows 7 and so on.
What if your running Norton or McAfee?
So wont all of these start conflicting?
Wont they slow down your systems?
How can you disable the ones you don?t need to be running?
Most of them don?t allow you to stop them or exit from them with out spending a lot of time digging around to fine them and adjust the properties some kind of way.
If your lucky enough to do that.
Was just wondering what everyone else thinks about all of this?
Thanks for the replys.