1. No you don't NEED one but you an get one later. Sygate is Free click here
Its like a bouncer for your PC. Nothing gets in or out without you knowing and giving permission.
2. Not really.
3. Windows Update & Defrag
4. YES. PC Advisor of course.
Remember if you have any questions then you can always ask us.
If you think you need to BUY software then ask us- we can usually suggest a freeware alternative that does the job just as good.
If you are not sure of any of the above - ask.