I will just give you what I use but it's just example

avg anti-v, comodo firewall...spybot s/d....Vista's Defender should also be on be default..key being "should" because it acts like backup. Running in tandem, just in case. Some people don't like it and drop it, but I like to keep it going with comodo, or whatever else I might be trying at the time. I also use spysweeper.
typically it's not a good idea to have 2 anti-v's loading up at the same time
well....where the disc came from, there might be a "coa" on the case. Where did this Vista come from? separate purchase or from Dell as an auto upgrade? The auto upgrades shouldn't really be around anymore I believe but just asking.